[chore] instruction for installing mava for developement

[sash-a]

What?

Added install instruction for people who want to develop for mava to readme.

Why?

I couldn't find this information easily and think it should be in the install instructions

How?

Changed the readme

[CLAassistant]

All committers have signed the CLA.

[codecov[bot]]

Codecov Report

Merging #803 (94cefe5) into develop (e523bb2) will increase coverage by 0.00%. The diff coverage is n/a.

:exclamation: Current head 94cefe5 differs from pull request most recent head b5b5a17. Consider uploading reports for the commit b5b5a17 to get more accurate results

@@           Coverage Diff            @@
##           develop     #803   +/-   ##
========================================
  Coverage    92.69%   92.70%           
========================================
  Files          150      150           
  Lines         8361     8357    -4     
========================================
- Hits          7750     7747    -3     
+ Misses         611      610    -1     

Impacted Files	Coverage Δ
mava/environment_loop.py	88.13% <0.00%> (-0.10%)	:arrow_down:
mava/systems/trainer.py	100.00% <0.00%> (ø)
mava/systems/parameter_server.py	100.00% <0.00%> (ø)
mava/utils/lp_utils.py	51.11% <0.00%> (+1.11%)	:arrow_up:

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