update README

README.md

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 # mpi_test
+
+Various standalone MPI binaries
+* all pass `argc`/`argv` to `MPI_Init`
+
+Adjust `Makefile` to match your environment, if needed
+* uses `mpicxx` and a few simple flags by default
+
+## Build
+```
+make
+```
+
+## Run all tests
+
+Adjust `run-all.sh` to match your environment, if needed
+```
+./run-all.sh
+```
+
+If any tests fail, you can run them individually.
+
+## Run individual tests
+
+Execute any binary you want using `mpirun`, or whatever is appropriate for your platform.