Add first time contributor guide

[jamievasales]

• Add a first time contributor guide and add it to the README.MD and the mkdocs

I myself am a first time contributor (both to the project and open source). I have made some assumptions so that something can be written but please do correct me on any assumptions that may be false. This is all quite meta 😄

[what-the-diff]

• Added a first time contributor guide
• Updated the readme to link to it
• Created documentation for it in mkdocs and added that too

[bodiam]

I like it! Not sure if we need two documents, but we can deal with the later! Thanks!