Proofreading edits

Add missing links (to references).
Minor edits throughout document.
Expand a few sentences in the 'Rationale' section
Update README.rst
Merge branch 'master' of https://github.com/theupdateframework/pep-maximum-security-model
Merge branch 'master' of https://github.com/vladimir-v-diaz/pep-maximum-security-model
Merge pull request #2 from vladimir-v-diaz/master

Proofread the PEP, fix a few issues with the text, and expand a paragraph in `Rationale`