Add SECRET_KEY instructions
Move database instructions up in `install.md`.

You have to run `python manage.py syncdb` before Celery will work.
Tidy up install docs
Add bookmarklet v1

I've built version one of the bookmarklet. You can see it on the "create link"
page at /manage/create/ (on the "develop" branch of the repo).

The interface is simple: the user clicks the button while viewing a page, and
Perma will automatically archive that page and generate a preview as usual.

Here's how it works on the backend:

- The bookmarklet passes data to
/service/bookmarklet-create/?v=1&url=http://something-to-archive.com/

- The view associated with that URL passes the query string data to
/manage/create/?v=1&url=http://something-to-archive.com/

- Some JavaScript on /manage/create/ reads the query string, populates the
"create" field, and submits the form
Don't autosubmit form for bookmarklet requests

Closing a CSRF vulnerability. See
https://github.com/harvard-lil/perma/commit/5c216fd2f7d61d57fa137070fe81940863bf99b1#commitcomment-5445045
added bookmarklet to tools and settings page