Add contributing guide

[Papooch]

If the project grows, we'll need contributing guidelines -> create a CONTRIBUTING.md file with steps.

[Papooch]

I added a file with basic information, but it definitely needs some polishing.

[micalevisk]

As a step on the contribution road, what do you think of using github issues forms like we have on nestjs repositories? I can open a PR to add them, I just need to know which forms and their fields do you would like to have

I can also add an hyperlink to the discord community for those that just want to chat about this lib

[Papooch]

Hey, thanks for the offer. I don't think I want a very rigid structure of the issue format as it often discourages people (me included) from opening it in the first place - and I think the project is not huge enough that it would benefit from it.

However, some guide on how to structure the issue in general would be helpful. A link to discord wouldn't hurt also.