[ Variables ] - Add missing documentation for repository variables (#…

…2032)

* docs: 📝 add vars instructions by cli and file loading

* docs: 📝 adjust varibles instructions

README.md

@@ -315,6 +315,14 @@ To run `act` with secrets, you can enter them interactively, supply them as envi
 - `act --secret-file my.secrets` - load secrets values from `my.secrets` file.
   - secrets file format is the same as `.env` format
 
+# Vars
+
+To run `act` with repository variables that are acessible inside the workflow via ${{ vars.VARIABLE }}, you can enter them interactively or load them from a file. The following options are available for providing github repository variables:
+
+- `act --var VARIABLE=somevalue` - use `somevalue` as the value for `VARIABLE`.
+- `act --var-file my.variables` - load variables values from `my.variables` file.
+  - variables file format is the same as `.env` format
+
 # Configuration
 
 You can provide default configuration flags to `act` by either creating a `./.actrc` or a `~/.actrc` file. Any flags in the files will be applied before any flags provided directly on the command line. For example, a file like below will always use the `nektos/act-environments-ubuntu:18.04` image for the `ubuntu-latest` runner: