Clarifications for Readme

[oldchili]

As suggested in this PR, We should probably add some clarification comments once there a Readme:

1. For why there is no way to exit MKR after shutdown, especially as the comment in the code about End.cage might make ppl think that ES is supported.

I would phrase it something like:
"Note that there is no support for freeing or redeeming MKR from the engine for vat.gem, and therefore no support for governance-less Emergency Shutdwon. In case there is a governance structure performing or concluding a shutdown process, the vat.gem accounting for this ilk can help perform a governance tokens distribution process."

2. Also comment something about the fact that during shutdown the oracle that is used might not represent the exact MKR market price (for example if it's a sticky oracle), and this can affect the MKR distribution to DAI holders.
   We can say that the MKR value is expected to fluctuate so much at that point anyway, so we don't consider this too impactful in comparison.