Update README, note on FOSS

[martinothamar]

Figured this migth as well be written down. Reasons:

• FOSS is great
• This library has a very reasonable scope/workload for a FOSS project
• I've had plenty use of FOSS in my career, nice to be able to give back
• Mediator pattern typically gets pretty deeply ingrained in application-layer code, so stability (public API, licensing, patching cadence) is very important IMO