Document Pycom's upstream tracking procedure.

[nevercast]

After #3 I noticed that it seems pycom-esp-idf does not directly track upstream, but that a force merge is applied after what I assume is a roll-back, and then fixes are applied on top.

Understanding how Pycom handles patching esp-idf while tracking espressif would help me better maintain my end.

My build process currently involves a tree of patch files and a tracking repo, currently that repo is here which I have just updated to the latest commit at pycom/pycom-esp-idf. This had to be a force push because the branches between old master and pycom's master branch had diverged (observe 3 commits ahead).

If Pycom is force updating and applying their patches on afterwards, I would greatly appreciate that process being exposed so that my patches might fit in to your already implemented system.

Kind regards
Josh