update README.md for BGC

Author: aekiss

README.md

@@ -1,5 +1,7 @@
-# 025deg_jra55_ryf
-Standard configuration for 0.25 degree [ACCESS-OM2](https://github.com/COSIMA/access-om2) experiment (ACCESS-OM2-025) with JRA55-do 1 May 1990 - 30 April 1991 repeat-year forcing (RYF9091).
+# 025deg_jra55_ryf with BGC
+Standard configuration for 0.25 degree [ACCESS-OM2](https://github.com/COSIMA/access-om2) experiment (ACCESS-OM2-025) with JRA55-do 1 May 1990 - 30 April 1991 repeat-year forcing (RYF9091) and coupled biogeochemistry in the ocean and sea ice.
+
+This is the BGC version, on the `master+bgc` branch. For the physics-only version (no BGC), use the `master` branch.
 
 For usage instructions, see the [ACCESS-OM2 wiki](https://github.com/COSIMA/access-om2/wiki).