Update README with license agreement notes

[backinaaz-blip]

Add additional notes about license agreements for CLDR tools.

CLDR-_____

• [x] This PR completes the ticket.

ALLOW_MANY_COMMITS=true

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}