Adjust the readme intro for the Beta1 release

[webmat]

We'll need to adjust the readme for this first release:

• Tone down the chances for breaking changes. The point of Beta1 is to start guaranteeing more backwards compatibility. But since it's Beta1, we shouldn't guarantee 100% necessarily yet.
• We should link to the specific git tag right in the intro, to make it easy to browse the repo as of the Beta1 release.
  • We should also ensure all internal links in the repo (e.g. towards use cases) are relative, and don't throw people back to master :-) (Make Markdown links relative, so browsing a git ref stays in the same git ref #173)
• Warning that in cases of discrepancies between the readme and schema.csv or template.json, the readme should prevail.
  • This is a temporary situation. Ideally all would be in line, but they won't be yet, at the time of Beta1
• Output the new version instead of 0.1.0. By the way, what should the Beta1 version be?
• Mention the two temporary removals (Temporarily remove the http object... #171 and Temporarily remove the user_agent field set... #172)?

[webmat]

I haven't seen mentions of breaking changes that needed to be toned down (referring to my first subtask above).

Other than this point, #174 is a first attempt at what this release could look like, from a readme/changelog point of view.

#173 will also be important for people to be able to navigate the markdown renderings directly on Github. Otherwise clicking some links brings them out of a git tag and back to the master repository, which is constantly in flux.