Add usage instructions

README.md

@@ -7,4 +7,16 @@ codeplug-prepare
 A simple helper utility to extract the keys for [codeplug][0]. This tool requires that you own a copy of MOTOTRBO CPS.
 
 
+Usage
+-----
+
+ * Download the pre-built binaries (link above) or build the project with MSBuild
+ * Download [OpenSSL](https://www.openssl.org/) (e.g. from [indy.fulgan.com](https://indy.fulgan.com/SSL/)) and extract it into the project folder
+ * Open a terminal (press <Win>+R and enter `cmd`)
+ * Navigate to the project folder (e.g. `cd C:\Users\Example\Downloads\codeplug-prepare`)
+ * Start the tool by entering `CodeplugPrepare`
+ * Execute the command involving `openssl`
+ * All done! You can now use the file `yourkey.pem` and the values of `CTB_KEY` and `CTB_IV` to [read and write codeplugs][0].
+
+
 [0]: https://github.com/george-hopkins/codeplug