Change md file names to README.md in each lesson subdirectory

**Is your feature request related to a problem? Please describe.**
When you navigate the lessons, the user clicks on the directory (i.e., Course Introduction), and is shown the contents of the directory. The user then must know to click on the markdown file to see the information.

**Describe the solution you'd like**
A simple change that would make the repo easier to use is to rename the markdown files in each lesson subdirectory to README.md. If you do that, when you click on the directory, the markdown will auomatically show, without having to select it.

**Describe alternatives you've considered**
It is fine as it is - most users will know what to do, but this change is simple and may make the repo easier to use.

**Additional context**
Just comment if you would like for me to make this change and I'll raise a PR.