Add 'developers' to the style guide (#118)

CONTRIBUTING.md

@@ -211,6 +211,12 @@ Other style points to be aware of are:
   fixed-width fonts where em dashes are not clearly visible.
 - Always say "Discourse Forum" and "Discord Chat" to avoid confusion between
   systems.
+- Prefer the term "developers" when talking about people who would write Carbon
+  code. We expect the Carbon's community to include people who think of
+  themselves using many titles, including software developers, software
+  engineers, systems engineers, reliability engineers, data scientists, computer
+  scientists, programmers, and coders. We're using "developers" to succinctly
+  cover the variety of titles.
 
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