add README for no-mpi branch of Darshan

README.nompi.md

+## Quick Start
+
+    $ cd darshan-serial/darshan-runtime
+    $ autoreconf -ivf
+    $ ./configure --with-log-path-by-env=PWD --with-jobid-env=RANDOM --with-mem-align=8 --disable-mpiio-mod --disable-stdio-mod --disable-pnetcdf-mod --disable-dxt-mod --disable-bgq-mod --disable-lustre-mod --without-mpi CC=gcc
+    $ make
+
+This will install serial Darshan in darshan-serial/install.  Then to use it,
+
+    $ LD_PRELOAD=/path/to/darshan-serial/darshan-runtime/lib/libdarshan.so DARSHAN_ENABLE_NONMPI=1 ./path/to/my.exe
+
+Your `my.exe` will run and create a Darshan log in `$PWD`.  Regular old
+`darshan-parser` can be used to parse the logfile.
+
+## Non-MPI Darshan
+
+This repository contains a version of Darshan that supports profiling in the
+absence of MPI at either runtime or compile time.  It can be built in two ways:
+
+1. **With MPI**, but capable of profiling non-MPI applications if linked against
+   them.  This is useful if a single build should work with both MPI and non-MPI
+   applications.
+2. **Without MPI** and capable of only profiling non-MPI applications.  This is
+   useful for building Darshan on systems that simply don't have MPI at all.
+
+At present, mode #2 only supports the POSIX module.  All other modules should be
+disabled at configure-time using:
+
+    ./configure --disable-mpiio-mod \
+                --disable-stdio-mod \
+                --disable-pnetcdf-mod \
+                --disable-bgq-mod \
+                --disable-lustre-mod \
+                --disable-dxt-mod \
+                --without-mpi \
+                ...
+
+Then to profile an application,
+
+    LD_PRELOAD=$PWD/lib/libdarshan.so DARSHAN_ENABLE_NONMPI=1 ./path/to/my.exe
+
+Do _not_ define `DARSHAN_ENABLE_NONMPI=1` globally, as this will create Darshan
+logs for _everything_ including basic commands like `ls`.