Update readme file to tell people to continue using 6.1.1 until a few bugs are fixed.

[ramonawalls]

We merged 6.2 to make editing more efficient, but it still has a few bugs.

Add the following to the readme file in the root directory:

"The 6.2 release of the MIxS repository is in transition and still has a few bugs. Until we remove this message from this readme file, please continue to use the MIxS 6.1.1 release, available at https://github.com/GenomicsStandardsConsortium/mixs/tree/mixs6.1.1."

If you have questions or concerns in the interm, please contact gensc-twg@groups.google.com or create an issue in this repository."