doxy: add a section about versioning.

libavutil/avutil.h

@@ -41,6 +41,39 @@
  * @li @ref lavu "libavutil" common utility library
  * @li @ref lavr "libavresample" audio resampling, format conversion and mixing
  * @li @subpage libswscale  color conversion and scaling library
+ *
+ * @section libav_versioning Versioning and compatibility
+ *
+ * Each of the Libav libraries contains a version.h header, which defines a
+ * major, minor and micro version number with the
+ * <em>LIBRARYNAME_VERSION_{MAJOR,MINOR,MICRO}</em> macros. The major version
+ * number is incremented with backward incompatible changes - e.g. removing
+ * parts of the public API, reordering public struct members, etc. The minor
+ * version number is incremented for backward compatible API changes or major
+ * new features - e.g. adding a new public function or a new decoder. The micro
+ * version number is incremented for smaller changes that a calling program
+ * might still want to check for - e.g. changing behavior in a previously
+ * unspecified situation.
+ *
+ * Libav guarantees backward API and ABI compatibility for each library as long
+ * as its major version number is unchanged. This means that no public symbols
+ * will be removed or renamed. Types and names of the public struct members and
+ * values of public macros and enums will remain the same (unless they were
+ * explicitly declared as not part of the public API). Documented behavior will
+ * not change.
+ *
+ * In other words, any correct program that works with a given Libav snapshot
+ * should work just as well without any changes with any later snapshot with the
+ * same major versions. This applies to both rebuilding the program against new
+ * Libav versions or to replacing the dynamic Libav libraries that a program
+ * links against.
+ *
+ * However, new public symbols may be added and new members may be appended to
+ * public structs whose size is not part of public ABI (most public structs in
+ * Libav). New macros and enum values may be added. Behavior in undocumented
+ * situations may change slightly (and be documented). All those are accompanied
+ * by an entry in doc/APIchanges and incrementing either the minor or micro
+ * version number.
  */
 
 /**