docs: Include minimal reference for templating mechanism

docs/variables.md

@@ -1,5 +1,33 @@
 # Workflow Variables
 
+Some fields in a workflow specification allow for variable references which are automatically substituted by Argo. 
+
+??? note "How to use variables"
+ Variables are enclosed in curly braces and **must not** include whitespace.
+
+ ``` yaml
+ apiVersion: argoproj.io/v1alpha1
+ kind: Workflow
+ metadata:
+ generateName: hello-world-parameters-
+ spec:
+ entrypoint: whalesay
+ arguments:
+ parameters:
+ - name: message
+ value: hello world
+ templates:
+ - name: whalesay
+ inputs:
+ parameters:
+ - name: message
+ container:
+ image: docker/whalesay
+ command: [cowsay]
+ # args: ["{{ inputs.parameters.message }}"] <- bad
+ args: ["{{inputs.parameters.message}}"] # good
+ ```
+
 The following variables are made available to reference various metadata of a workflow:
 
 ## All Templates

mkdocs.yml

@@ -14,6 +14,8 @@ google_analytics:
 markdown_extensions:
  - codehilite
  - admonition
+ - pymdownx.superfences
+ - pymdownx.details
  - toc:
  permalink: true
 nav: