docs: Update README.md & add CONTRIBUTING.md

[cleptric]

This is based on our Laravel SDK CONTRIBUTING.md.

Please let me know if I should add more things or if anything in here is plain wrong.

Closes #1289

[ste93cry]

I wonder if we should really enter the rabbit hole of how to install PHP, rather than just saying that it is a prerequisite for the working environment (of course)

[stayallive]

I agree, we shouldn't provide a step-by-step installation guide for PHP plenty of info on that on the internet, but we can give some guidance on where to get it. Maybe some inspiration can be taken from the Laravel SDK where I updated the guide few months ago:

• https://github.com/getsentry/sentry-laravel/blob/master/CONTRIBUTING.md
• Improve README.md and CONTRIBUTING.md sentry-laravel#548

[cleptric]

I aligned the CONTRIBUTING.md with our Laravel SDK.
I added our branch workflow as well as a note about updating the CHANGELOG.md file.
Once this is merged, let's also update sentry-laravel and sentry-symfony.

[vladanpaunovic]

I agree with above sentiments from @stayallive and @ste93cry (thanks folks!).

Other than that, this looks good. Nice addition to this repo! Thanks @cleptric! 🥳

[cleptric]

Based on #1289, I made some changes to the README.md as well.

[stayallive]

Looks good, we can always keep iterating on this 😄