readConcern:

parsing has been already added to ReadConcernArgs,
1. To src/mongo/s/strategy.cpp
add code that verifies that level:snapshot is set in a command that is a part of at transaction.
if not must return

ErrorCodes::InvalidOptions

error

2. add a testcase to sharding suite that verifies that
a) level:snapshot can be parsed by mongos
b) level:snapshot is only allowed when there is a txnNumber and lsid - i.e its allowed only as a part of a transaction
c) level:snapshot is permitted only for the find, aggregate, count, distinct commands. - this is not going to be checked on mongos but is verified on mongod with a supportsReadConcern call

There are geoNear, group, geoSearch and parallelCollectionScan commands that may support it as well if it will be free but its not required.