Update the README with submit and compiler/mpi

utils/matrix/README.md

@@ -41,6 +41,44 @@ Instructions
    ```shell
    ./utils/matrix/setup_matrix.py -f matrix.cfg
    ```
+   Optionally use the `--submit` flag to submit jobs once each configuration
+   has been built and set up.
 
-8. The matrix build doesn't take care of running the jobs on a compute node.
+9. The matrix build doesn't take care of running the jobs on a compute node.
    You will need to do that yourself.
+
+Matrix of compilers and MPI libraries
+-------------------------------------
+
+You control the matrix of compilers and MPI libraries by how you call
+```shell
+./conda/configure_compass_env.py ...
+```
+This is because the configure script knows about which compilers and MPI 
+libraries are  available for a given machine, something tricky to figure out in 
+`./util/matrix/setup_matrix.py` directly.
+
+You can do:
+```shell
+./conda/configure_compass_env.py --compiler all --mpi all ...
+```
+to get all supported compilers and MPI libraries.  You can do:
+```shell
+./conda/configure_compass_env.py --compiler intel --mpi all ...
+```
+to get just the `intel` configurations.  You can do:
+```shell
+./conda/configure_compass_env.py --compiler all --mpi openmpi ...
+```
+to get just the `openmpi` configurations or:
+```shell
+./conda/configure_compass_env.py --compiler intel ...
+```
+(omitting the `--mpi`) to get the default MPI library for intel.  Finally, you
+can give a list of compilers and a list of the same length of MPI libraries,
+such as:
+```shell
+./conda/configure_compass_env.py --compiler intel intel intel gnu \
+    --mpi impi openmpi mvapich mvapich ...
+```
+This will give you 3 intel variants and a gnu variant.