Executing_Tests.rst: Improve getting started part

[dglez]

For short term contributors: we understand that getting your commits well
defined like we require is a hard task and takes some learning. If you
look to help without wanting to contribute long term there's no need
for you to learn this. Just drop us a message and we'll take care of brushing
up your stuff for merge!

Checklist

• I read the commit guidelines and I've followed
  them.
• I ran coala over my code locally. (All commits have to pass
  individually. It is not sufficient to have "fixup commits" on your PR,
  our bot will still report the issues for the previous commit.) You will
  likely receive a lot of bot comments and build failures if coala does not
  pass on every single commit!

After you submit your pull request, DO NOT click the 'Update Branch' button.
When asked for a rebase, consult coala.io/rebase
instead.

Please consider helping us by reviewing other peoples pull requests as well:

• pick up any PR at https://coala.io/review
• review it (check https://coala.io/reviewing for more info)
• if you are sure that it needs work, use cobot mark wip <URL> to get it out
  of the review queue.

The more you review, the more your score will grow at coala.io and we will
review your PRs faster!

[prnvdixit]

@dglez Don't submit PR from any other existing branch - Make your own branch. Go through https://coala.io/git and https://coala.io/newcomer if you haven't already 😄.
Create a new PR anyway and mention the issue you are trying to solve too - Again, read https://coala.io/newcomer first before doing anything else. 👍

[prnvdixit]

Any maintainer, please Close this PR.