Update main Readme for project with links to info about each map

[egrguric]

Update https://github.com/polyrhythm-project/polyrhythm-maps/blob/master/README.md to add in links to the Readme for each map:

• Composers https://github.com/polyrhythm-project/polyrhythm-maps/blob/master/composers/README.md
• Corpus https://github.com/polyrhythm-project/polyrhythm-maps/blob/master/corpus/README.md
• Timeline https://github.com/polyrhythm-project/polyrhythm-maps/blob/master/timeline/README.md

[egrguric]

Quickly added links to each Readme. Main Readme needs a bit more information about general file structure as per: library-rc.github.io/rdm/content/03_create_readme.html.

[bel28kent]

Here is a link to the Google Doc I have been writing the documentation on.

https://docs.google.com/document/d/1LR19gyfLX9Cs1-96zJl-JLosPqwx40xkVM-CHvmu9vk/edit?usp=sharing

[egrguric]

@bel28kent will update access to link to be fully open and will save PDF copy in main repository folder.

[bel28kent]

I have updated access to the link, and made Craig and Dr. Poudrier editors on the document.

[epoudrier]

Add link to documentation on READme file. /

[epoudrier]

Improve readability of directory and files listing: specify meaning of bold and reduce indentation. @bel28kent