We split the section about pull requests, reducing the steps needed to
contribute since there's already a link to help people working on their
first pull request, and the rest don't need to be told to fork the repo.
Current team members don't use twitter very often, the `Not-ready` label
has has been removed, and issue assignees are sometimes deceiving.
We're also removing the "Limpiar" section from the Spanish document,
which had already been removed from the English document in commit
8eb8bb9a.
Why:
* Try to make the repo more friendly for first time contributors and any contributor on general
How:
* Adding a PRs-welcom badget that links to the issues list filtered by PRs-welcome label
* Adding a Link to on the Contributing guide to a free course about Pull Request and Contributions on Github.
