Contribution documentation

[ChunkyProgrammer]

I think it'd be useful to add Contribution documentation to Invidious to make it easier for people to develop for invidious.

Example content:

The following packages need to be installed onto your system for invidious to function:

• crystal
• sqllite3

Install dependencies shard install

Run Invidious crystal run src/invidious.cr

[unixfox]

Following the normal installation in the current documentation works too for development purpose.

But feel free to submit a new page for the documentation about how to start developing for Invidious.