Bump API version more & add VERSIONING.md.

Recent changes on master have changed the public API in a backwards-
incompatible way. The next release will therefore be 2.0.0, not 1.4.0.

Add a VERSIONING.md for reference.

Closes mypaint#92.

VERSIONING.md

@@ -0,0 +1,43 @@
+# API and ABI versioning policy
+
+libMyPaint cannot afford to be sloppy about its version numbers because
+other projects depend on it.
+
+API versions on `master` increment ahead of the upcoming release
+as and when features and changes are added. ABI versions are always
+updated immediately before each release, and _only_ then.
+
+1. The `master` branch contains the _future release_'s API version
+   number. This version number is updated as and when new features and
+   API changes are committed to the master branch. Do this by updating
+   [configure.ac][]'s `libmypaint_api_*` macros as part of your commit.
+   The reference point for these updates is the current stable release's
+   version number.
+
+   * Rules for the API version number: libmypaint uses
+     [Semantic Versioning][].
+
+   * Note that the major and minor components of the API version number
+     form part of the _soname_ for binary library files used at runtime,
+     but they're not part of the `libmypaint.so` symlink used by your
+     compiler when it tries to find a dynamic library to link against.
+
+2. We use prerelease suffixes at the end of the API version number
+   during active development to help you identify what you are building
+   against. The API version number of a formal release normally does not
+   contain any prerelease sufffix.
+
+3. For ABI versioning, libmypaint does exactly what the GNU docs say. We
+   will always update some part of the ABI version number immediately
+   before each public release. This is done by tweaking
+   [configure.ac][]'s `libmypaint_abi_*` macros. ABI versions are _only_
+   bumped immediately before a release, and maintain their own (somewhat
+   weird) version sequence that's independent of the API version number.
+
+   * Rules for the ABI version number: refer to
+     “[Updating library version information][]”
+     in the GNU libtool manual.
+
+[configure.ac]: ./configure.ac
+[Semantic Versioning]: http://semver.org/
+[Updating library version information]: https://www.gnu.org/software/libtool/manual/html_node/Updating-version-info.html

configure.ac

@@ -2,8 +2,8 @@
 AC_PREREQ(2.62)
 
 # API version: see https://github.com/mypaint/libmypaint/wiki/Versioning
-m4_define([libmypaint_api_major], [1])
-m4_define([libmypaint_api_minor], [4])
+m4_define([libmypaint_api_major], [2])
+m4_define([libmypaint_api_minor], [0])
 m4_define([libmypaint_api_micro], [0])
 m4_define([libmypaint_api_prerelease], [alpha])  # may be blank
 # The platform version is "major.minor" only.