Update instructions are mysql-specific. Added a line for Postgres users

Signed-off-by: David Greaves <david.greaves@jollamobile.com>

Checking the redis version will warn users that
are using an old version to update. Included
reference to the troubleshooting guide.

Removing duplication by putting everything in the readme. Allow us to close issues that deviate from the guidelines.

Better header and detail why you need two times the storage. Also moving memory and storage to the bottom, OS is more important.

Most of these are comments but a few are strings for users.

Might be an idea to run this from time to time:
https://github.com/lyda/misspell-check

It runs mostly clean now.

* updated system hooks documentation and code comments
* fixed access to system hooks if no user given resulting in a `500 Server Error`
* added tests

All the info to return codes from the API functions are available in the `README.md` file as suggested.

Allow connection to Redis via unix socket, using
unix:/var/run/redis/redis.sock for example.

Default behaviour does not change, except that the full Redis URL must
be configured, with redis:// for tcp or unix: for unix socket.