default port matches endpoint; documentation improvements

[Aptimex]

Default port for the configure command (i.e. --port) will now be set to whatever port was specified by the --endpoint command (instead of 51820), since that argument is already required and they usually need to be the same anyway.

Default route for the configure command is now a useful 0.0.0.0/0 instead of a useless 0.0.0.0/32, and --route is no longer a required argument (closes #35). Added a note to the README about how this default may create networking issues in some cases.

Various documentation improvements:

• Server config is not compatible with other wireguard tools
• Made more clear that clients can freely change their AllowedIPs
• The endpoint and API addresses must be for the same server in the add server command
• Servers will forget stuff that was configured via the API if they restart
• Clarified limitations of clients added to non-first-hop servers (or not all of them)
• Improved visual clarity of the example network diagrams

[Aptimex]

@luker983 Can you check that I didn't say anything incorrect in the README changes? Other general thoughts are appreciated too

[luker983]

Approved.