Switch to semantic versioning 2.0.0 (semver.org)

- Changes "1.0" to "1.0.0" and bumps the "Last modified" date in
  the specification working document.
  NOTE: the "(Draft)" supplement is not part of semantic versioning
- Mentions use of semantic versioning in the spec's readme

Author: lukpueh

README.rst

@@ -1,7 +1,7 @@
 The Update Framework specification
 ----------------------------------
 
-Latest: `version 1.0 <https://github.com/theupdateframework/specification/blob/master/tuf-spec.md>`_
+Latest: `version 1.0.0 (Draft) <https://github.com/theupdateframework/specification/blob/master/tuf-spec.md>`_
 
 
 Contact
@@ -27,6 +27,11 @@ Please see `LICENSE-MIT.txt
 and `LICENSE-APACHE.txt
 <https://github.com/theupdateframework/specification/blob/master/LICENSE-APACHE.txt>`_.
 
+Versioning
+----------
+
+The TUF specification uses `Semantic Versioning 2.0.0 <https://semver.org/>`_
+for its version numbers.
 
 Acknowledgements
 ----------------

tuf-spec.md

@@ -1,8 +1,8 @@
 # <p align="center">The Update Framework Specification
 
-Last modified: **19 June 2018**
+Last modified: **13 September 2019**
 
-Version: **1.0 (Draft)**
+Version: **1.0.0 (Draft)**
 
 We strive to make the specification easy to implement, so if you come across
 any inconsistencies or experience any difficulty, do let us know by sending an