doc: document protocol versioning policy

In order to communicate with developers of alternative implementations
and be reasonable about compatibility, we need to assign versions to
the protocol too. This commit documents the protocol versioning policy.

doc/VERSIONING.md

@@ -1,5 +1,7 @@
 # Versioning
 
+## Package versioning
+
 When we reach 1.0, we will just adhere to [semantic
 versioning](http://semver.org/) strictly, but semver says nothing about
 versions `<1.0.0`. Thus we extend semver's rules applying them to these
@@ -10,3 +12,10 @@ a feature or a bugfix.
 Accordingly, if labels named `semver-major` or `semver-minor` are added to any
 issue or pull request before we have released `v1.0.0`, they actually assume
 minor and patch subversions.
+
+## Protocol versioning
+
+Protocol versions are bound to the first version of the `metarhia-jstp` package
+that implements the specific version of the JSTP protocol. When the package
+reaches 1.0.0, the protocol version will unconditionally bumped to 1.0.0 too,
+regardless of whether `metarhia-jstp@1.0.0` brings any changes to the protocol.