Mention command-line flags in user's guide.

docs/source/usersguide/install.rst

@@ -330,6 +330,20 @@ Alternatively, you could run from any directory:
 Note that in the latter case, any output files will be placed in the present
 working directory which may be different from ``/home/username/somemodel``.
 
+Command-Line Flags
+------------------
+
+OpenMC accepts the following command line flags:
+
+-g, --geometry-debug   Run in geometry debugging mode, where cell overlaps are
+                       checked for after each move of a particle
+-n, --particles N      Use *N* particles per generation or batch
+-p, --plot             Run in plotting mode
+-r, --restart file     Restart a previous run from a state point or a particle
+                       restart file
+-s, --threads N        Run with *N* OpenMP threads
+-v, --version          Show version information
+
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 Configuring Input Validation with GNU Emacs nXML mode
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