Add contributing docs to README

[Andrew-Chen-Wang]

Add contributing docs to README

• Also adds hacking as an optional dependency for others to more easily install

[Andrew-Chen-Wang]

I see this might be a duplicate of #119

[gi0baro]

@Andrew-Chen-Wang thank you for your contribution.
As you correctly discovered, this looks a duplicate of #119, which is opened by quite a long time, my bad.

To give you my personal ideas on this:

• I'm more open to a make command for building a local environment and installing maturin, rather than hacking into project dependencies
• I'm considering it might worth to actually produce a contribution guide in a separated file (eg: CONTRIBUTING.md) using a similar approach to what Pallets did on Flask and link that document in the readme, rather than bloating the readme with a long paragraph.. WDYT?

[Andrew-Chen-Wang]

agree. docs/CONTRIBUTING sounds like a good place with a hyperlink to the file in the README