doc: document versioning policy

This commit documents the pre-1.0 versioning policy as a Markdown file.

Refs: #38
PR-URL: #42

doc/VERSIONING.md

@@ -0,0 +1,12 @@
+# 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
+versions this way: in `0.minor.patch` scheme `minor` acts as a major semver
+version and `patch` as both minor and patch, regardless of whether a change is
+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.