Add section to main README pointing to all the test tables

[pgleeson]

It would be good if a brief motivation for testing across the engines of Biosimulators, as well as a list of tests performed/links to tables, was added to the main repo README file. This would be a good entry point for anyone who wants to look at the compatibility across the different simulation engines and types of models tested, without having to click through all the folders.

[stellaprins]

Done in PR #105

[pgleeson]

Done!