Add Equality Act 2010 reason to collect gender or sex data

src/patterns/gender-or-sex/index.md

@@ -12,7 +12,9 @@ This pattern explains how to ask users about gender or sex.
 
 ## When to use this pattern
 
-You should only ask users about gender or sex if you genuinely cannot provide your service without this information.
+You should only use this design pattern to ask users about gender or sex if you genuinely cannot provide your service without this information. 
+
+You should collect information about gender or sex, and other protected characteristics under the Equality Act 2010, to comply with the public sector Equality Duty using the separate [Equality information pattern](/patterns/equality-information/).
 
 If you do need to ask, use ‘sex’ when you need biological data (for example, if you’re providing a medical service). In all other cases, use ‘gender’.