Clean-up README

[m-mohr]

Some proposals for a cleaner README:

• Directly link to examples and JSON Schema directly so that they are easy to find (the schema wasn't linked to at all from the spec, the spec itself wasn't linked to at all from the repo README)
• Update version numbers to one consistent value instead of 0.4.0 and 0.5.0-dev in different places
• Markdown tweaks, e.g. code fences
• Remove "Optional" as it is in STAC to make it easier to see the difference directly. Only required fields are declared, all other are implicitly optional.
• Don't hide links somewhere in the text

[tschaub]

Thanks, @m-mohr