Documentation fixes and clarification

- In README, refer to “install.rst”, not “install.html”
- In rapi.rst, wrap line longer than 72 characters
- In rlib2.py, update and clarify description of POST vs. PUT

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>