Add README.md file.

Add a dedicated README.md to the KeePassX repository to explain the
purpose of KeePassX, and to inform it's audience as to how they can
contribute.
Remove ModifiedOnExpandedStateChanges config option.

I'm pretty sure noone knew what it actually does.
This is the sort of option users shouldn't be bothered with.
Merge branch 'hbetts-readme'

https://github.com/keepassx/keepassx/pull/46
Wrap overly long lines in README.md.
Small README corrections.
Expand the build-dependency section of the README.
Add a translations section to the README.