Write a tutorial for getting started with pull requests for php-src

[ericpoe]

Questions to answer could include:

• Which repo do I fork?
• What is a good way to format and word a PR so that it is more likely to be accepted?
• Which php-src directories do I pay attention to? Which do I leave alone?
• How do we mark our tests or PRs to show that they are part of PHP TestFest?
• How do we mark our tests or PRs to show that we are part of a specific group for PHP TestFest?