Add a README file

[GabrielMajeri]

Since we plan on leaving the plugin's code public, we should create a README file in the root directory, to describe what this plugin is, who is it intended for, what changes we have made etc.

Possible sections:

• Description
• Version of OJS tested with
• Plugin installation instructions
• Development/contributing instructions
• Licensing

[GabrielMajeri]

Might not be necessary to do this anymore in this repo, since I've ported all of the plugin's code to the corresponding changes in the fork.

That being said, we should still write-up this information, but it could be added to this file: https://github.com/GabrielMajeri/ojs/blob/custom/README.md