Grammar mistakes and typo in README.md

[FancyNeuron]

There are a few grammatical errors in the README.md and a typo

Fix the following lines:

• Teams allow people -> Teams allows people - noun plural, no "s" in verb
• those groups of people -> those groups - we have already established in the line above, that this is about groups of people
• sharing purpose -> sharing purposes
• CireclesManager -> CirclesManager - typo