Add contributing documentation

[jelhan]

Add a CONTRIBUTING.md. It should contain all relevant information to ease contributions to this repository. Especially it should explain:

1. The ESJ template located in templates/.github/workflows/ci.yml used to generate the workflow.
2. The concept of parser that extract the configuration from existing files (currently only TravisCI configuration is supported through src/utils/parse-travis-ci-config.js).
3. Snapshot testing with jest: How it works, how to update a snapshot (yarn test -u) and how to verify that changes are as expected.
4. Fixtures located in tests/fixtures used as test scenarios.

See #15 (comment) as an example of a question that should be answered by contributing docs.