Add "How To Set Session Limits" guide

[yondonfu]

We have observed quite a lot of orchestrators returning OrchestratorCapped errors and it is likely that these orchestrators are using the default -maxSessions value (10). If this is the case, then the orchestrators' transcoding capacity is being under utilized. We should add a guide for setting session limits that can use livepeer_bench.

The initial approach for setting session limits might involve:

1. Running livepeer_bench with an increased # of concurrent sessions with a fixed/common ABR ladder until the ratio of video time to transcode time is <= X. The suggestion for X might be to set X to 1.2 to target 20% faster than real-time to give a buffer for network transit.
2. Given an approximation upload/download bandwidth, using the input/output bitrates of a fixed/common ABR ladder estimate the # of sessions that can be handled
3. Take the lower of 1 vs. 2

The above would just be a rough way to get started setting session limits and we would want to note that the approach assumes a fixed/common ABR ladder, but you could follow the same steps given different ABR ladders as well.