Create README.md

Run directory for the SEAC4RS code.   

I removed terpene chemistry since it is too uncertain, and removed Eloise’s SOA chemistry for simplicity.
I also removed the tracking species for OH, and the second MACR photolysis channel per J.Mao rec.

I added the fast photolysis of carbonyl nitrates from Chris M., reduced the yield of IEPOX from RIP + OH, 
and updated the HPALD products so they are just HOx and CO.

I added PAN, RCHO, SO2, SO4, NH3, OCPO, BCPO, HAC, GLYC ,NO2, HNO2 as emitted species in input.geos
and added ACTA and HCOOH as tracers.

That should be all!
~Katie