update README

README.md

@@ -5,7 +5,53 @@ proprietary tools (such as cryptlib).
 
 ## Usage
 
-./apc-p15-tool --keyfile ./key.pem --certfile ./cert.pem
+Currently the tool contains two commands: create and install. The tool 
+can be run with the --help flag to see options.
 
-Command outputs ./apctool.p15 which can be directly loaded on to an 
+i.e. `./apc-p15-tool --help`
-APC NMC2 (Network Management Card 2).
+
+Help can also be run on a subcommand to see the options for that 
+subcommand.
+
+e.g. `./apc-p15-tool install --help`
+
+### Create
+
+Create creates an apc p15 file from given key and cert pem files or 
+content.
+
+e.g. `./apc-p15-tool create --keyfile ./apckey.pem --certfile ./apccert.pem`
+
+The command outputs ./apctool.p15 by default. This file can be 
+directly loaded on to an APC NMC2 (Network Management Card 2).
+
+### Install
+
+Install works similarly to create except it doesn't save the p15 file 
+to disk. It instead uploads the p15 file directly to the specified 
+remote host, via scp.
+
+e.g. `./apc-p15-tool install --keyfile ./apckey.pem --certfile ./apccert.pem --apchost myapc.example.com:22 --username apc --password someSecret --fingerprint 123abc`
+
+## Note About Install Automation
+
+The application supports passing all args instead as environment 
+variables by prefixing the flag name with `APC_P15_TOOL`. 
+
+e.g. `APC_P15_TOOL_KEYPEM`
+
+Additionally, there is a second binary built with just the install
+command so the subcommand is not needed.
+
+There are mutually exclusive flags that allow specifying the pem 
+as either filenames or directly as strings. The strings are useful 
+for passing the pem content from another application without having 
+to save the pem files to disk.
+
+Putting all of this together, you can combine the install binary with 
+a tool like LeGo CertHub (https://www.legocerthub.com/) to call the 
+install binary, with environment variables, to directly upload new 
+certificates as they're issued by LeGo, without having to write a 
+separate script.
+
+![LeGo CertHub with APC P15 Tool](https://raw.githubusercontent.com/gregtwallace/apc-p15-tool/main/img/apc-p15-tool.png)