Clarification of supported characters in variable names (#2737)

Author: felixi-octopus

src/pages/docs/projects/variables/getting-started.mdx

@@ -162,6 +162,29 @@ If you are using the [Structured configuration variables](/docs/projects/steps/c
 
 ## Variable Recommendations
 
+### Use standard characters in variable names
+Prefer using standard characters when naming variables. While it's technically possible to include almost any character, **variable names** with non-standard characters can only be accessed in limited ways - such as via the `$OctopusParameters` dictionary. Variable values can include any character.
+
+| Variable name  | Value                                  |
+| ---------------| -------------------------------------- |
+| `WontWork`     | `#{Octopus.Action[StepA].Output.Foo!}` |
+| `Works`        | `#{Octopus.Action[StepA].Output.Ba-r}` |
+ 
+**Step A**
+```ps
+Set-OctopusVariable -name "Foo!" -value "FooValue"
+Set-OctopusVariable -name "Ba-r" -value "BarValue"
+```
+
+**Step B**
+```ps
+$OctopusParameters["Works"]                             # Returns: BarValue
+$OctopusParameters["WontWork"]                          # Returns: #{Octopus.Action[StepA].Output.Foo!}, not FooValue
+$OctopusParameters["Octopus.Action[StepA].Output.Foo!"] # Returns: FooValue
+```
+
+To ensure your variable can be accessed via Octostache expressions, stick to alphanumerics, periods (`.`), hyphens (`-`), underscores (`_`), and spaces for the **name** of the variable.
+
 ### Group variables into Variable Sets
 
 Variables sometimes naturally fit into groups, and often you'll find multiple applications will use these groups.  Octopus has [Variable Sets](/docs/projects/variables/library-variable-sets); allowing you to group variables into a reusable set so that they can be used by other projects.