Docs for asv

[Vaibhav-Chopra-GT]

Description

Added README.md file in the benchmarks directory with contents from CONTRIBUTING.md. I think it would be good to have these contents in both of these files for future users and developers. Should these contents be also available or linked somewhere in PyBaMM's documentation website?
Fixes # (issue)

Type of change

• New feature (non-breaking change which adds functionality)
• Optimization (back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)

Key checklist:

• No style issues: $ flake8
• All tests pass: $ python run-tests.py --unit
• The documentation builds: $ cd docs and then $ make clean; make html

You can run all three at once, using $ python run-tests.py --quick.

Further checks:

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works