Add pre.erb docs to the readme

CHANGELOG.md

@@ -1,5 +1,7 @@
 ## HEAD
 
+- Add: Rundoc command `pre.erb` command used for dynamically templating any command using ERB syntax. ()
+
 ## 4.0.0
 
 - Add: Rundoc command `background.stdin_write` to send a string to a backtround process' STDIN. This allows driving REPL interfaces (https://github.com/zombocom/rundoc/pull/79)

README.md

@@ -73,6 +73,8 @@ This will generate a project folder with your project in it, and a markdown `REA
 - Execute Bash Commands
   - [$](#shell-commands)
   - [fail.$](#shell-commands)
+- Dynamic command templating
+  - [pre.erb](#pre.erb)
 - Printing
   - [print.text](#print)
   - [print.erb](#print)
@@ -257,6 +259,66 @@ These custom commands are kept to a minimum, and for the most part behave as you
 
 Running shell commands like this can be very powerful, you'll likely want more control of how you manipulate files in your project. To do this you can use the `file.` namespace:
 
+## Dynamic command templating
+
+Meta commands that produce no output but instead allow for generating commands via dynamic templates.
+
+Current Commands:
+
+- `pre.erb`
+
+### pre.erb
+
+Template other commands using `pre.erb` in front of them. This will process the command using [ERB](https://rubyapi.org/3.3/o/erb), Ruby's built in templating library, then run the resulting command.
+
+For example:
+
+  ```
+  :::>> pre.erb $ echo "The answer to everything is <%= 6*7 %>"
+  ```
+
+When this runs, it will first replace the template with the result of the ERB. It would be the same as this:
+
+    ```
+    :::>> $ echo "The answer to everything is 42"
+    ```
+
+The binding (variable and method scope) for `pre.erb` is shared across all executions and the default `print.erb` command. That means you can use it to persist data or logic and re-use it:
+
+    ```ruby
+    :::-- print.erb <%
+      # Won't be rendered because it's using `--` visibility
+      def lol
+        "haha"
+      end
+
+      user = "Schneems"
+    %>
+    ```
+
+    ```
+    :::>> pre.erb $ echo <%= user %> said <%= lol() %> | tr '[:lower:]' '[:upper:]'
+    ```
+
+When run this would produce:
+
+    ```
+    $ echo Schneems said haha | tr '[:lower:]' '[:upper:]'
+    SCHNEEMS SAID HAHA
+    ```
+
+Multi line commands are also supported
+
+    ```
+    :::>> pre.erb file.write "lol.txt"
+    Super secret key:
+      <%= "#{key}" %>
+    ```
+
+The only thing to watch out for is if the resulting template contains a `:::>>` (or similar) rundoc marker at the beginning of the line, Rundoc will think it is a new command rather than a part of `pre.erb` template.
+
+The visibility of the `pre.erb` is forwarded to whatever command is run.
+
 ## Print
 
 Current commands: