doc: simplify contributing guidelines and improve the visibility to contributors

[haowang-bioinfo]

Main improvements in this PR:

• improve the contributing guidelines in Human-GEM
  • remove the section of git commands, which are no longer necessary after the implementation of yaml workflow
  • increase visibility of contributing section in README by moving it to top
  • move annotation file details from main README to that under model directory

I hereby confirm that I have:

• Tested my code on my own computer for running the model
• Selected develop as a target branch

[mihai-sysbio]

This looks nice. I have left some minor suggests here and there, feel free to adapt/reject.

[JonathanRob]

Thanks for these nice changes, @Hao-Chalmers

Since we're on the topic of the README, can I suggest that the tables in the Reaction, Metabolite, and Gene Annotations section are moved to a separate README within the model directory (where the annotation files reside)? The reason is that they make the README pretty long, and such detail may not be necessary in the main README.

[haowang-bioinfo]

@JonathanRob good idea, go ahead (in this branch)