Add documentation and csv format

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 # Spandx
 
 A ruby API for interacting with the https://spdx.org software license catalogue.
+This gem includes a command line interface to scan a software project for the
+software licenses that are associated with each dependency in the project.
+`spandx` leverages an offline cache of software licenses for known dependencies.
+The offline cache allows spandx to perform a truly airgap friendly scan of software
+projects.
 
 ![badge](https://github.com/mokhan/spandx/workflows/ci/badge.svg)
 
@@ -22,6 +27,46 @@ Or install it yourself as:
 
 ## Usage
 
+### Command line interface
+
+The command line interface supports operations to build and fetch the latest offline index.
+See the help for each subcommand for more information on how to use the command.
+
+```bash
+モ spandx
+Commands:
+  spandx help [COMMAND]      # Describe available commands or one specific command
+  spandx index [SUBCOMMAND]  # Manage the index
+  spandx scan LOCKFILE       # Scan a lockfile and list dependencies/licenses
+  spandx version             # spandx version
+```
+
+To scan a specific project file use the `scan` command:
+
+```bash
+モ spandx scan dotnet/application.sln
+モ spandx scan java/pom.xml
+モ spandx scan python/Pipfile.lock
+モ spandx scan ruby/Gemfile.lock
+```
+
+To activate airgap mode use the `--airgap` option:
+
+```bash
+モ spandx scan dotnet/application.sln --airgap
+モ spandx scan ruby/Gemfile.lock --airgap
+```
+
+Airgap mode assumes that an offline cache has been placed in `$HOME/.local/share/`.
+
+To fetch the latest offline cache:
+
+```bash
+モ spandx index fetch
+```
+
+### Ruby API
+
 To fetch the latest version of the catalogue data from [SPDX](https://spdx.org/licenses/licenses.json).
 
 ```ruby