Added info for new caption option

Author: mblawrence

README.md

@@ -36,6 +36,7 @@ Options
 
 You have a couple of formatting options via attributes of the fenced code block to control the rendering
 
+- Pandoc caption and automatic image numbering - Use `{.mermaid caption="Caption Text Here"}`
 - Image Format - Use `{.mermaid format=svg}`     Default is png
 - Width  - Use `{.mermaid width=400}`     default with is 500
 - Theme - Use `{.mermaid theme=forest}` default is 'default'. Corresponds to `--theme`  flag of mermaid.cli
@@ -56,4 +57,4 @@ JSON and CSS configuration
 
 Mermaid cli allows you to specify additional options in a json configuration file and a css file. `mermaid-filter`
 will look in the current directory for `.mermaid-config.json` and `.mermaid.css` and if found, pass them in to 
-mermaid cli.
+mermaid cli.