Add documentation for the custom gradle parameters

doc/gradle.md

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+# Custom gradle parameters
+
+You can use these parameters by specifying them inside the `gradle.properties` file as 
+`systemProp.<name>=<value>` or passing them through the CLI with `-D<name>=<value>`.
+
+## packageSuffix
+This allows you to specify a suffix, which will be added on release builds to the application id, 
+the `app_name` string and the apk file.  
+No validation is made, so make sure to pass in a valid value.
+
+## skipFormatKtlint
+This allows you to skip the `formatKtLint` task.  
+No value is needed.   
+It is used for CI in order to check for badly formatted files.