Readme.MD improvement suggestion

[henkvancann]

Three points after installing keripy based on Readme.md:

1. Went smoothly after some interpretation of the text

Python3.9.1+ should be installed on a Mac using the Python.org installer according to https://installpython3.com/mac/
libsodium is a homebrew case

2. Some commands in code markup are mandatory to be executed in the Readme some are not. Maybe it good to indicate this distinction?

3. Another hint for the readme is to make it more Noob-resistant:

"start the commands from the keripy root dir" which is obvious for a programmer, but might not be for a tester.

Well done, in general it went really smooth to get it going. Can't wait to dump it in a Docker container ;)