Document Instructions and various network's integrations

Tracking issue: https://github.com/serai-dex/serai/issues/57

docs/integrations/Monero.md

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+# Monero
+
+### Addresses
+
+Monero addresses are an enum, defined as follows:
+
+  - `standard`:   32-byte key, 32-byte key.
+  - `subaddress`: 32-byte key, 32-byte key.
+  - `featured`:   1-byte flags, 32-byte key, 32-byte key.
+
+This definition of Featured Addresses is non-standard given the flags are
+intended to be a VarInt, yet as of now, only half of the bits are used, with no
+further planned features. Accordingly, it should be fine to fix its length,
+which makes it comply with expectations present here. If needed, another enum
+entry for a 2-byte flags Featured Address could be added.
+
+### In Instructions
+
+Monero In Instructions are present via `tx.extra`, specifically via inclusion
+in a `TX_EXTRA_TAG_PADDING` tag, and accordingly limited to 255 bytes.
+
+### Out Instructions
+
+Out Instructions ignore `data`.