Added Public API and Versionning Policy to readme file.

Readme.md

@@ -95,6 +95,35 @@ To do this, you must set the `empty_element_suffix` variable of the parser
 object to ">".
 
 
+Public API and Versionning Policy
+---------------------------------
+
+Version numbers are of the form *major*.*minor*.*patch*.
+
+The public API of PHP Markdown consist of the two parser classes `Markdown`
+and `MarkdownExtra`, their constructors, the `transform` and `defaultTransform`
+functions and their configuration variables. The public API is stable for
+a given major version number. It might get additions when the minor version
+number increments.
+
+**Protected members are not considered public API.** This is unconventionnal 
+and deserves an explanation. Incrementing the major version number every time 
+the underlying implementation of something changes is going to give nonsential 
+version numbers for the vast majority of people who just use the parser. 
+Protected members are meant to create parser subclasses that behave in 
+different ways. Very few people create parser subclasses. I don't want to 
+discourage it by making everything private, but at the same time I can't 
+guarenty any stable hook between versions if you use protected members.
+
+**Syntax changes** will increment the minor number for new features, and the 
+patch number for small corrections. A *new feature* is something that needs a 
+change in the syntax documentation. Note that since PHP Markdown Lib includes
+two parsers, a syntax change for either of them will increment the minor 
+number. Also note that there is nothigng perfectly backward-compatible with the
+Markdown syntax: all inputs are always valid, so new features always replace
+something that was previously legal, although generally non-sensial to do.
+
+
 Bugs
 ----