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 +12,18 @@ 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
+
+Before the package reaches version 1.0.0, the protocol version is bound to the
+first of the `metarhia-jstp` package versions that implement the same
+specification of the protocol. It would be nice to have it versioned
+independently, but it's too late to do it, since we haven't been versioning it
+from the beginning and we have made changes to the specification multiple
+times.
+
+When the package reaches 1.0.0, the protocol version will be unconditionally
+bumped to 1.0.0 too, regardless of whether `metarhia-jstp@1.0.0` brings any
+changes to the protocol.  We will then take this as a starting point and the
+protocol specification will be versioned with semver independently from the
+package.