Rename `master` to `main` and communicate the change :unicorn:

[Arghya-Banerjee]

I have tried to create and rename the default branch as "main" as you requested. Please look at it if it works. Thank You

Fixes #459

[carmenbianca]

Thank you for your contribution @Arghya-Banerjee !

I think this change is best done after the v0.15.0 milestone. So this will probably stay in the fridge for a little while :)

[floriansnow]

@carmenbianca What's your concern if we were to do this now?

[linozen]

Here is some guidance by GitHub