Add docs about RecordRTC vs Pipe

[becky-gilbert]

Even though researchers shouldn't have to know anything in particular about our switch from Pipe to RecordRTC for the Experiment Runner (lookit/ember-lookit-frameplayer#307), it might be a good idea to write some documentation for researchers about it. In particular we should (a) note the commit SHA for the last Pipe version, in case researchers need to switch back to Pipe, and (b) document any changes to the data logging (either content of the logs or interpretations of values such as timestamps and stream times).