Update BSON_README.md

Completed unfinished statement about which settings turn off reading and writing of splits information
Added Lazy BSON as a format type
Verify that there is a valid output URI set before trying to write any records HADOOP-81
move environment-specific settings to the top of script, set in bash vars
round-robin target shards by re-ordering splits/chunks for sharded collections.
Use github-flavored markdown so we get syntax highlighting.
add more syntax highlighting to readme
Serialization and Deserialization tests done