Add client/server version requirements to readme

[jbyrnessnip]

trafficstars

Add a section to the readme that describes version requirements between client and server. In other words, given a server version, which clients are compatible.

[jbornemann]

@sagarsane @jdigger I guess we could be painfully explicit in the readme about how we follow semantic versioning?

[jdigger]

Except that there can certainly be a bump in the major version without being any change in the protocol. A prime example would be v3.x and v4.x being protocol compatible but different versions for AEM version changes...