Clarify spec for version

[pombredanne]

We some key issues to tackle wrt. version encoding:

• Clarifications, in particular about version encoding #35 - Must space ' ' in a version be percent-encoded?
• Percent encoding spec and : and / #39 - Must ':' in a version be percent-encoded?
• test: character encoding #364 - Must ':' in a version (e.g., 'dockerimage@sha256:244f...') be percent-encoded?

And this PR:

• Add RFC 2119/8174 note and convert terms to uppercase as needed #373

[pombredanne]

@mprpic This is yours since your volunteered. Do you want to join the project?

[mprpic]

@pombredanne Yes please.