[Docs] Add version guarantees (#2882)

Make some version guarantees.

We also need to do a pass over the existing things this would cover and decide if any of them should be made private.

docs/version_guarantees.rst

@@ -0,0 +1,36 @@
+.. _version-guarantees:
+
+==========
+Versioning
+==========
+
+Red is versioned as ``major.minor.micro``
+
+While this is very similar to SemVer, we have our own set of guarantees.
+
+Major versions are for project wide rewrites and are not expected in the foreseeable future.
+
+==========
+Guarantees
+==========
+
+Anything in the ``redbot.core`` module or any of its submodules 
+which is not private (even if not documented) should not break without notice.
+
+Anything in the ``redbot.cogs`` module or any of it's submodules is specifically
+excluded from being guaranteed.
+
+If you would like something in here to be guaranteed,
+open an issue making a case for it to be moved.
+
+=======================
+Breaking Change Notices
+=======================
+
+Breaking changes in Red will be noted in the changelog with a special section.
+
+Breaking changes may only occur on a minor or major version bump.
+
+A change not covered by our guarantees may not be considered breaking for these purposes, 
+while still being documented as a breaking change in internal documentation
+for the purposes of other internal APIs.