Documentation for what all is additional in this boilerplate

**Describe the task**
This package provides a lot of things on top of Bootstrap. There should be a documentation of what all is it.

**Expected behavior**
A docs folder with one or more markdown files. They will cover:
1. Line heights
2. Font sizes
3. Min/max values
4. Background positioning
5. Refer the [utilities.scss](https://github.com/ColoredCow/bootstrap-boilerplate/blob/master/assets/scss/utilities.scss) for more
6. Refer the [variables.scss](https://github.com/ColoredCow/bootstrap-boilerplate/blob/master/assets/scss/variables.scss) for more

These markdown files inside the docs folder will be later used to create the GitHub pages.