Add a note on GitHub that this is superseded by the plugins for existing editors and IDEs!

[mfhepp]

As per:

https://forum.micropython.org/viewtopic.php?p=19802&sid=980a89b47bcad067a5486536804dd86a#p19802

Please add a short note about your decision to stop development of PyMakr prominently in the README of the Github repository at https://github.com/pycom/Pymakr. It does not make sense to have people spend their time with a dead tool...