Points plaintext links to the correct targets in the README.

This pull request for README.md updates the "Get involved" links for:

- forum (was broken, now points to forum)
- self-referential link to the github site (was broken, now points to GH openFrameworks repo)
- contributing guideline (was not in markdown format)

I did not inline the links with the introductory term (e.g. `[forum](http:...)` so that the href targets are obvious in a search and as to not deviate much from the current layout of the page.