Added index to an overwhelmingly large README.md

[mardab]

Hello,

I find your port of NuttX to Cortex-A53+GICv2 invaluably amazing, which is sadly what I cannot say about README file of this project.

It is understandable to want to include every piece of information needed to work with this port, but at over 11 thousand lines, about half of them being exemplary log output, this approaches an Applicaton Note or Datasheet territory.

Without being sure what should be left in README and what should be moved to a different document (perhaps a Github-provided project wiki?) for full readability, the least I can do is to add an index to every part of README, which as you can see is over 100 entries with indentation inferred from content.

Should you accept this improvement even partially, I will be looking forward to provide "real" contributions to this fascinating project.

Sincerely,
Marcin Dąbrowski

[lupyuen]

Thanks! Really appreciate it, I'll update the Table of Contents for future updates. Please check out the articles, I think they might be better for collaboration.