MAINT: Added LICENSE and updated README

[Schefflera-Arboricola]

As suggested by @rossbar in the last nx-community meeting I've added the installation instructions in the readme. I updated the readme with a few other changes as well. I also added the CC0 1.0 Universal license. LMK know what you think of that. Also, I tried following through the instructions in RELEASE.md but I couldn’t bcoz I don’t have the GH_TOKEN

RuntimeError: You need to set the environment variable `GH_TOKEN`. The token is used to avoid rate limiting, and can be created at https://github.com/settings/tokens.

And I decided to remove the link to the _dispatchable decorator's docs because it will probably be moved around in the future and instead put the backend doc's page's link.

Thank you :)

[jarrodmillman]

Why not use a BSD-3 license?

[Schefflera-Arboricola]

Why not use a BSD-3 license?

I'm not an expert in this but I choose to go with CC0 1.0 Universal license because I know that it has the least restrictions, also nx-guides use it so I thought maybe projects other than the main networkx project(in networkx org) use this license. Let me know if the BSD-3 or BSD-3-Clause or any other license would be more appropriate for this project and I'll change it. thanks!

[rossbar]

FWIW the reason nx-guides uses a CC license instead because the majority of the content is narrative text instead of code. I think BSD-3 is more appropriate here - just my 2 cents!

[dschult]

That is my understanding as well -- that CC is better for prose/text while BSD is better for code.
So, let's match the NetworkX license rather than the nx-guides license. :)

[dschult]

Looks good oto me!