feat: GPL V3 License

[AshtonStephens]

Description

• Add GPL License
• Add GPL License badge to README.md

[netrome]

Should we also add copyright headers to all source files like we do in stacks-core?

[AshtonStephens]

I've never understood that, isn't the license enough?

[netrome]

I've never understood that, isn't the license enough?

Don't know, but I assume the copyright headers are required because why else are they included everywhere in GPL3-licensed code? Could be worth looking into.

[netrome]

Yeah it seems to be a requirement. From https://www.gnu.org/licenses/gpl-howto.en.html:

Here's a brief summary of what you need to do to release a program under one of our licenses:

Get a copyright disclaimer from your employer or school.
Give each file the proper copyright notices. Make sure [to clearly identify which versions of the license users can use](https://www.gnu.org/licenses/identify-licenses-clearly.html).
Add a COPYING file with a copy of the GNU GPL or GNU AGPL.
Also add a COPYING.LESSER file with a copy of the GNU LGPL, if you use that.
Put a license notice in each file.
(Optionally) make the program display a startup notice.
(If using the AGPL) make the program offer copies of its source code.

This involves adding two elements to each source file of your program: a copyright notice (such as “Copyright 1999 Terry Jones”), and a statement of copying permission, saying that the program is distributed under the terms of the GNU General Public License (or the Lesser GPL, or the Affero GPL).