Started basic command-line usage docs.

docs/index.rst

@@ -18,6 +18,7 @@ User Documentation
    :maxdepth: 1
 
    installation
+   usage
    interactive
    logging
    samples

docs/usage.rst

@@ -0,0 +1,32 @@
+==================
+Command-line Usage
+==================
+
+Peregrine can be used as a stand alone command-line tool for processing a
+sample data file into a set of PVT solutions.
+
+Usage information can be obtained by running::
+
+  $ peregrine --help
+
+At its most basic, peregrine can simply be passed the name of a sample data
+file to process and it will perform acquisition, tracking and navigation
+solution on the data file::
+
+  $ peregrine path/to/sample_data_file
+
+After each stage (acquisition, tracking and navigation solution), a 'results'
+file is saved with the same name as the data file with the extensions
+``.acq_results``, ``.track_results`` and ``.nav_results`` appended. These files
+can be used for analysis at a later point or can be used to re-run peregrine,
+skipping stages by loading their results from disk.
+
+For example, to skip acquisition specify the ``-a`` option::
+
+  $ peregrine -a sample_data_file
+
+and peregrine will attempt to load the acquisition results from the file
+``sample_data_file.acq_results`` and skip straight to tracking.
+
+Similar options ``-t`` and ``-n`` exist for tracking and navigation respectively.
+