Add docs on env variables

Author: danielfdsilva

packages/client/README.md

@@ -10,10 +10,20 @@ See root README.md for instructions on how to install and run the project.
 
 Some client options are controlled by environment variables. These are:
 ```
+# App config
+## Title and description of the app for metadata
 APP_TITLE
 APP_DESCRIPTION
+
+# API
+## If the app is being served in from a subfolder, the domain url must be set.
+PUBLIC_URL
 REACT_APP_STAC_BROWSER
 REACT_APP_STAC_API
+
+# Theming
+REACT_APP_THEME_PRIMARY_COLOR
+REACT_APP_THEME_SECONDARY_COLOR
 ```
 
 You must provide a value for the `REACT_APP_STAC_API` environment variable. This should be the URL of the STAC API you wish to interact with.