Docs: Add documentation for EPEE Portable Storage

Ripped directly from @jtgrassie 's monero-binary-rpc repo. It's a very helpful little document and I think it deserves a place in the main repo.

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+# Portable Storage Format
+
+## Background
+
+Monero makes use of a set of helper classes from a small library named
+[epee](https://github.com/monero-project/monero/tree/master/contrib/epee). Part
+of this library implements a networking protocol called
+[Levin](https://github.com/monero-project/monero/blob/master/contrib/epee/include/net/levin_base.h),
+which internally uses a storage format called [Portable
+Storage](https://github.com/monero-project/monero/tree/master/contrib/epee/include/storages).
+This format (amongst the rest of the
+[epee](https://github.com/monero-project/monero/tree/master/contrib/epee)
+library), is undocumented - or rather relies on the code itself to serve as the
+documentation. Unfortunately, whilst the rest of the library is fairly
+straightforward to decipher, the Portable Storage is less-so.  Hence this
+document.
+
+## Preliminaries
+
+### String and integer encoding
+
+#### varint
+
+Varints are used to pack integers in an portable and space optimized way. The
+lowest 2 bits store the amount of bytes required, which means the largest value
+integer that can be packed into 1 byte is 63 (6 bits).
+
+| Lowest 2 bits | Size value    | Value range                       |
+|---------------|---------------|-----------------------------------|
+| b00           | 1 byte        | 0 to 63                           |
+| b01           | 2 bytes       | 64 to 16383                       |
+| b10           | 4 bytes       | 16384 to 1073741823               |
+| b11           | 8 bytes       | 1073741824 to 4611686018427387903 |
+
+#### string
+
+These are simply length (varint) prefixed char strings.
+
+## Packet format
+
+### Header
+
+A packet starts with a header:
+
+| Header        | Type      | Value                 |
+|---------------|-----------|-----------------------|
+| Signature     | 8 bytes   | 0x0111010101010201|   |
+| Version       | byte      | 0x01                  |
+
+### Section
+
+Next we have a root object (or section as the library calls it). This is a map
+of name-value pairs called [entries](#Entry). It starts with a count:
+
+| Section       | Type      |
+|---------------|-----------|
+| Entry count   | varint    |
+
+
+Which is followed by the section's name-value [entries](#Entry) sequentially:
+
+### Entry
+
+| Entry             | Type                  |
+|-------------------|-----------------------|
+| Name              | string<sup>1</sup>    |
+| Type              | byte                  |
+| Count<sup>2</sup> | varint                |
+| Value(s)          | (type dependant data) |
+
+
+<sup>1</sup> Note, the string used for the entry name is not prefixed with a
+varint, it is prefixed with a single  byte to specify the length of the name.
+This means an entry name cannot be more that 255 chars, which seems a reasonable
+restriction.
+
+<sup>2</sup> Note, this is only present if the entry type has the array flag
+(see below).
+
+#### Entry types
+
+The types defined are:
+
+```cpp
+#define SERIALIZE_TYPE_INT64                1
+#define SERIALIZE_TYPE_INT32                2
+#define SERIALIZE_TYPE_INT16                3
+#define SERIALIZE_TYPE_INT8                 4
+#define SERIALIZE_TYPE_UINT64               5
+#define SERIALIZE_TYPE_UINT32               6
+#define SERIALIZE_TYPE_UINT16               7
+#define SERIALIZE_TYPE_UINT8                8
+#define SERIALIZE_TYPE_DUOBLE               9
+#define SERIALIZE_TYPE_STRING               10
+#define SERIALIZE_TYPE_BOOL                 11
+#define SERIALIZE_TYPE_OBJECT               12
+#define SERIALIZE_TYPE_ARRAY                13
+```
+
+The entry type can be bitwise OR'ed with a flag:
+
+```cpp
+#define SERIALIZE_FLAG_ARRAY              0x80
+```
+
+This signals there are multiple *values* for the entry. When we are dealing with
+an array, the next value is a varint specifying the array length followed by
+the array item values. For example:
+
+<p style="padding-left:1em; font:italic larger serif">name, type, count,
+value<sub>1</sub>, value<sub>2</sub>,..., value<sub>n</sub></p>
+
+#### Entry values
+
+It's important to understand that entry *values* can be encoded any way in which
+an implementation chooses. For example, the integers can be in either big or
+little endian byte order.
+
+Entry values which are objects (i.e. `SERIALIZE_TYPE_OBJECT`), are stored as
+[sections](#Section).
+
+Note, I have not yet seen the type `SERIALIZE_TYPE_ARRAY` in use. My assumption
+is this would be used for *untyped* arrays and so subsequent entries could be of
+any type.
+
+## Monero specifics
+
+### Entry values
+
+#### Strings
+
+These are prefixed with a varint to specify the string length.
+
+#### Integers
+
+These are stored little endian byte order.
+
+#### Hashes, Keys, Blobs
+
+These are stored as strings, `SERIALIZE_TYPE_STRING`.
+
+#### STL containers (vector, list)
+
+These can be arrays of standard integer types, strings or
+`SERIALIZE_TYPE_OBJECT`'s for structs.
+
+#### Links to some Monero struct definitions
+
+- [Core RPC
+  definitions](https://github.com/monero-project/monero/blob/master/src/rpc/core_rpc_server_commands_defs.h)
+- [CryptoNote protocol
+  definitions](https://github.com/monero-project/monero/blob/master/src/cryptonote_protocol/cryptonote_protocol_defs.h)
+
+
+
+[//]: # ( vim: set tw=80: )