Improve README.md and CONTRIBUTING.md

For better experience for new contributors we should improve our "how to get started" documentation inside the repository. (Basically the files `README.md` and `CONTRIBUTING.md`.)

Cool would be that a new developer can find all the information she needs on how to set up her local environment, run tests and start to contribute.

It has been done already for [sentry-python](https://github.com/getsentry/sentry-python) so we can steal a lot of stuff from over there.

This is how those files should look like (If the section is not applicable to the SDK, we can ommit it.)

### README.md
- Sentry header (if not already there copy from sentry-python)
- badges of coverage etc for different versions/integrations (if available)]
- Headline1: Getting started
- Headline2: Requirements (describe if there are any requirements that need to met in order for the SDK to work.)
- Headline2: Install (how to install the SDK to the local dev environment)
- Headline2: Configuration (how to add the SDK to your project)
- Headline2: Usage (how to verify configuration, enriching stuff and so on.)
- Headline1: Integrations (list of all integrations (if any) with links to documentation)
- Headline1: Migrate from {old version name} (if there is a old raven version or some other incompatible old version)
- Headline1: Contributing to the SDK (link to [CONTRIBUTING.md](http://contributing.md/))
- Headline1: Getting Help/Support (describing how to get help/support. just copy text from sentry-python repository)
- Headline1: Resources (links to places on the web)
- Headline1: License (link to license file. see sentry-python repository)

(as a template on how this should look like see: [https://github.com/getsentry/sentry-python/blob/master/README.md](https://github.com/getsentry/sentry-python/blob/master/README.md))

### CONTRIBUTING.md
- intro text and link to how to contribute to documentation (copy from sentry-python repository)
- Headline1: How to report a problem (copy from sentry-python repository)
- Headline1: Submitting changes (copy from sentry-python repository)
- Headline1: Development Environment (how to set up, step by step guide. should be as easy as possible so that novice devs can do it. should work when copy pasting into shell on a fresh computer)
- Headline1: Running Tests (step by step guide. should work when copy pasting into shell on a fresh computer)
- Headline1: Releasing a new Version (copy from and ammend from sentry-python if this SDK also uses craft for deployment)
- Headline2: Versioning Policy (copy from sentry-python, if this SDK also uses semver)
- Headline1: Commit message guideline (link to docs, copy from sentry-python)

(as a template on how this should look like see: [https://github.com/getsentry/sentry-python/blob/master/CONTRIBUTING.md](https://github.com/getsentry/sentry-python/blob/master/CONTRIBUTING.md))

Start with README.md and CONTRIBUTING.md, then compare both with the sentry-python templates linked in the issue. Verify the repository’s actual requirements, local setup, test workflow, contribution process, support resources, release guidance, and license details before documenting them. Done means a new contributor can follow the files to set up, test, and begin contributing.

Written by the indexing model from the issue text.