Make "Create a Good PR" tutorial

[U8NWXD]

This tutorial should be comprehensive, and then https://github.com/oppia/oppia/wiki/Make-a-pull-request can become the same content in a reference-like format. The tutorial could reference sections of https://github.com/oppia/oppia/wiki/Make-a-pull-request as well and talk more about "how to follow those instructions" (e.g. how to write a good PR description).

[LittleRainC]

Hi, may I work on this? I'm going to link a new page like this under "Make your first PR". Here is how it looks like:

[seanlip]

@LittleRainC This is really quite nice, and I like the positive framing! I'm assigning you, feel free to go ahead and make a PR for it. Thanks!

[LittleRainC]

Thanks! PR been created -> #457

[LittleRainC]

@seanlip Hi, I have made PR, let me now if it is good when you get a chance.