model parameter and makefile for compilation

[sadath-12]

What's new

• This PR adds a new parameter to the freeedcom/ai-codereviewer Github Action: OPENAI_MODEL, which enables its users to customize the OpenAI model as needed.

• Updated readme with more detailed contributing guide

• Added Makefile for single command compilation

With these updates, it'll be possible to use another OpenAI model e.g. gpt-3.5-turbo instead and compile the code with command make compile

[bbernstein]

There is another pr open for just changing the model and it defaults to gpt-4. That may be a better default since that is the version described in the README file in a few places as well as the "About" of the github repo.

Also, not a biggie, but this got reformatted with different code editor settings than the original. Perhaps this repo could use a .eslintrc or something like that so formatting is consistent.

[villesau]

Thanks for the contribution! This was addressed already in the other PR that was just merged. Don't hesitate to fix some further issues you might face though :)