doc/technical/elemental-deviation: new doc file

Document protocol changes between vanilla TS6 and Elemental-ircd TS6.
The changes are very minimal and will not likely affect day to day
performance of the IRC daemon, but are still worth mentioning.

doc/technical/elemental-deviation.markdown

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+## Elemental-ircd specific deviations from the normal TS6 protocol
+
+As a general rule of thumb and for compatibility reasons, protocol changes are
+made as infrequently and minimally as possible. However there are some things
+that have been changed in how elemental handles the server to server protocol
+that are worth mentioning.
+
+For readability's sake, any lines that a server sends are prefixed by `>>>` and
+any lines it expects to recieve are prefixed by `<<<`.
+
+### Changes
+
+#### Channel owner mode
+
+If enabled, a new mode will be added to the channel prefix list. This mode (+y)
+has a prefix of "~" and in most IRC services packages is used to signify the
+owner of a specific channel.  If enabled, it will show up in SJOIN lines as such:
+
+    >>> :[sid] SJOIN [channel-ts] #opers + :~[ts6-uid]
+
+A more practical example would be:
+
+    >>> :420 SJOIN 13840003504 #opers + :~@42000000A
+
+As the mode for this channel is `+y`, all `TMODE` changes adding or removing `+y`
+from a client in a channel should set and unset this mode accordingly. An example
+of adding the mode follows:
+
+    >>> :47GAAAAAB TMODE 13840003504 #opers +y 42000000A
+
+Handling removal is similar.
+
+Currently, the only way to detect if a remote server supports these extended
+modes is to remotely query a server's `VERSION` and read out the `005` reply.
+