add environment_variable.validate to bashly doc

Author: DannyBen

lib/bashly/docs/env.yml

@@ -49,6 +49,16 @@ environment_variable.required:
   help: Specify that this variable is required.
   url: https://bashly.dannyb.co/configuration/environment-variable/#required
   example: |-
+    environment_variables:
     - name: api_key
       help: Your API key
       required: true
+
+environment_variable.validate:
+  help: Apply custom validation functions.
+
+  url: https://bashly.dannyb.co/configuration/environment-variable/#validate
+  example: |-
+    environment_variables:
+    - name: build_dir
+      validate: dir_exists