Document row level headers

This requires alphagov/tech-docs-gem#214 to be
released first - the current version of the tech docs template doesn't
support row headers in markdown tables.

Alternatively we could document that you need to use HTML tables to set
row level headings.

source/write_docs/content/index.html.md.erb

@@ -37,12 +37,21 @@ You should include `<div style="height:1px;font-size:1px;">&nbsp;</div>` before
 <div style="height:1px;font-size:1px;">&nbsp;</div>
 ```
 
+Add more columns and rows as needed.
+
 You can change the alignment of the text:
 
 - Centre: `|:---:|`
 - Right: `|---:|`
 
-Add more columns and rows as needed.
+You can create row headings by prefixing the content of the cell with `# `:
+
+```bash
+|Column header|Column header|
+|:---|:---|
+|# Row header|content|
+|# Row header|content|
+```
 
 ### Create table using HTML