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.

If a Travis-CI account was created, the following could be added underneath the KeePassX title:
```
[![Build Status](https://travis-ci.org/keepassx/keepassx.svg)](https://travis-ci.org/keepassx/keepassx)
```