New name of i-PI main branch

[yairl...@gmail.com]

Hi everyone,

We are really close to the release of i-PI v3.0, and
 we will use this opportunity to rename the main branch from 'master' to 'main'.
Since this means that ongoing developments branched out from 'master' might have a painful merging process, we would like to give you a warning message in advance.

If you are an i-PI user, keep going as usual.

If you are an i-PI developer, we strongly advise you to create a PR within the next 2 weeks so we can merge your implementation smoothly. If this time frame doesn't work for you, please communicate with use as soon as possible (either replying to this message, writing to yl...@cam.ac.uk or joining the i-PI slack channel here)

Best regards,
Yair