Add doc

docs/website/versioned_docs/version-3.0.0/command-reference/deploy.md

@@ -64,3 +64,10 @@ components:
                 - name: main
                   image: {{CONTAINER_IMAGE}}
 ```
+
+## Sustituting variables
+
+The Devfile can define variables to make the Devfile parameterizable. The Devfile can define values for these variables, and you 
+can override the values for variables from the command line when running `odo deploy`, using the `--var` and `--var-file` options.
+
+See [Sustituting variables in odo dev](dev.md#sustituting-variables) for more information.

docs/website/versioned_docs/version-3.0.0/command-reference/dev.md

@@ -43,6 +43,17 @@ In the above example, three things have happened:
 You can press Ctrl-c at any time to terminate the development session. The command can take a few moment to terminate, as it
 will first delete all resources deployed into the cluster for this session before terminating.
 
+### Sustituting variables
+
+The Devfile can define variables to make the Devfile parameterizable. The Devfile can define values for these variables, and you 
+can override the values for variables from the command line when running `odo dev`, using the `--var` and `--var-file` options.
+
+The `--var` option takes a comma-separated list of `variable=value` pairs, where `=value` is optional. If the `=value` is omitted, the value is extracted from the environment variable named `variable`. In this case, if the environment variable with this name is not defined, the value defined into the Devfile will be used.
+
+The `--var-file` option takes a filename as argument. The file contains a line-separated list of `variable=value` pairs, with the same behaviour as before.  
+
+Note that the values passed with the `--var` option overrides the values obtained with the `--var-file` option.
+
 ## Devfile (Advanced Usage)