Update Working Groups page

[oruebel]

It would be nice to add the working groups for the various pending enhancement proposals to the list e.g., ndx-microscopy. We should also list ndx-events as a past working group.

[rly]

I created #73 to add info on the NWBEP001 Review Working Group. The WG for NWBEP004 (optical devices and microscopy) has not been formed yet - it is still being reviewed by the TAB.