License and readme updates

[kraftbj]

• Add license.txt Fixes Add license.txt #1
• Update readme to include notice of plugin deactivation.

This PR adds a license.txt consisting of:

1. current copyright of code remains with the contributors who contribute, similar to WordPress itself. The WordPress Foundation, wordpress.org, etc do not ask contributors to waive their copyright.
2. previous copyright of the forked code. I can not find a previous copyright statement in the code that was forked, so it is a best effort to indicate copyright belongs to the previous project owners and/or the contributors.
3. The GPL v2 itself.

To the readme, a couple of instances are added to indicate the this plugin will deactivate previous versions of ACF/ACF Pro/SCF. Previously (and still the case) that an admin notice is presented upon activating this plugin of which plugin was deactivated on a given site.

Lastly, it updates the version numbers as 6.4.1-beta3, the next beta version of this new version so these changes are included on the wordpress.org repo.