Add a Macros chapter (#137)

Pitch thread: https://forums.swift.org/t/64339

Fixes: rdar://107494884

Author: amartini51

Style.md

@@ -98,6 +98,11 @@ which means it’s not always suitable for generating a parser for Swift code.
 
 # Terms and Rules
 
+## attribute names
+
+In the guide, write `@` before attribute names;
+in the reference, omit it.
+
 ## back deploy
 
 Spelled as two words, not closed up or hyphenated.
@@ -122,6 +127,13 @@ you should generally try to avoid them.
 Deeply nested headings often indicate
 that there’s a better way to organize the content.
 
+## macro names
+
+In the guide,
+write `@` before the name when referring to attached macros
+and `#` before the name when referring to freestanding macros.
+In the reference, omit it.
+
 ## memberwise initializer
 
 Not hyphenated as “member-wise”.