Add spec version- and release mgmt instructions

- Add gitflow-based version- and release management instructions to
the Versioning section of the README.

- Add links to the "latest stable", "current draft", "new changes
since latest stable" and "release history" of the specification to
the header of the README document.

Note that the version number of the latest stable version of the
specification is no longer shown in the README, which eliminates
the need to update the README with every new release.

Author: lukpueh

README.rst

@@ -1,7 +1,10 @@
 The Update Framework specification
 ----------------------------------
 
-Latest: `version 1.0.0 <https://github.com/theupdateframework/specification/blob/master/tuf-spec.md>`_
+- `latest stable <https://github.com/theupdateframework/specification/blob/master/tuf-spec.md>`_
+- `current draft <https://github.com/theupdateframework/specification/blob/draft/tuf-spec.md>`_
+- `new changes since latest stable <https://github.com/theupdateframework/specification/compare/master..draft>`_
+- `release history <https://github.com/theupdateframework/specification/tags>`_
 
 
 Contact
@@ -31,7 +34,33 @@ Versioning
 ----------
 
 The TUF specification uses `Semantic Versioning 2.0.0 <https://semver.org/>`_
-for its version numbers.
+(semver) for its version numbers, and a gitflow-based release management:
+
+- The 'master' branch of this repository always points to the latest stable
+  version of the specification.
+- The 'draft' branch of this repository always points to the latest development
+  version of the specification and must always be based off of the latest
+  'master' branch.
+- Contributors must submit changes as pull requests against these branches,
+  depending on the type of the change (see semver rules).
+- For patch-type changes, pull requests may be submitted directly against the
+  'master' branch.
+- For major- and minor-type changes, pull requests must be submitted against
+  the 'draft' branch.
+- Maintainers may, from time to time, decide that the 'draft' branch is ready
+  for a new major or minor release, and submit a pull request from 'draft'
+  against 'master'.
+- Before merging a branch with 'master' the 'last modified date' and 'version'
+  in the specification header must be bumped.
+- Merges with 'master' that originate from the 'draft' branch must bump either
+  the major or minor version number.
+- Merges with 'master' that originate from any other branch must bump the patch
+  version number.
+- Merges with 'master' must be followed by a git tag for the new version
+  number.
+- Merges with 'master' must be followed by a rebase of 'draft' onto 'master'.
+
+
 
 Acknowledgements
 ----------------