Document originating repository terminology

[rgaiacs]

Related with swcarpentry/shell-novice#738 and carpentries/styles#215.

Instructions to novices

1. Create a section called terminology at https://github.com/swcarpentry/lesson-example/blob/gh-pages/_episodes/06-style-guide.md
2. Describe the use of the term "originating repository".

[limako]

For my new instructor homework, I'm looking at old "help wanted" issues for something to work on. This is so old, I wanted to check to make sure it's still what people want to do -- it looks pretty straightforward to do, which is why I wonder why no-one has done it.

I noticed that "master" is used in an earlier episode of the lesson-example https://github.com/carpentries/lesson-example/blob/gh-pages/_episodes/02-tooling.md

Since I'm new a this, should I issue separate pull requests? Or can/should I do them both together?