Rename files from `index.md` to `readme.md`

[prashantrahul141]

The lesson markdown files are currently named index.md, which means that when you open a lesson, you have to open the index.md file to start reading. If it were renamed to readme.md instead, GitHub would automatically detect it and display it right below the file section, just like it does in the root directory.

I can open a pr for this! Opened an issue instead, because I wanted to make sure it is something everyone wants.

[Sahil12524]

Great idea btw!

[n0toose]

What if there were a table of contents in the README (plus cross-linking, i.e. "click here for the next lesson") instead?

[prashantrahul141]

@n0toose We could have both. A table of contents, cross linking in each lesson AND files being named readme.md.