revise contributor list

[mbjones]

Consider contributors for the 2.2 release. The main active contributors were jones, o'brien, mecum, maier, cjones, boettiger. Maybe others that I missed...

[mobb]

add shild

[mbjones]

Add @twhitaker

[twhiteaker]

My info would be:
, Tim Whiteaker (whiteaker@utexas.edu)

[mbjones]

I think contributors are now complete in the README. Please review, this issue will be closed soon.

[mbjones]

TODO:

• Add @srearl to the contributors