Add style entry for "back deploy".

This will be needed when we write docs for the backDeployed(upTo:)
attribute from SE-0376: Function Back Deployment.

Co-authored-by: Chuck Toporek <chuck_toporek@apple.com>

Author: amartini51

Style.md

@@ -98,6 +98,10 @@ which means it’s not always suitable for generating a parser for Swift code.
 
 # Terms and Rules
 
+## back deploy
+
+Spelled as two words, not closed up or hyphenated.
+
 ## compiler, the
 
 See entry for *Swift*.