Update README.md with more complete API docs.

- Added description for "goal" and "elsewhere" fields.
- Made a note that `%participant_id` is [currently](https://github.com/gittip/www.gittip.com/issues/503) case sensitive.

Here's how the API section of README.md looks like after this PR:
https://github.com/shurcooL/www.gittip.com/tree/patch-1#api