Add rule about TC39 proposals that haven't reached stage 3

There has been some confusion around which language features should be
used and which should not be used yet. We have a pretty clear-cut rule
at Airbnb about this, which is do not use any TC39 proposals that have
not reached stage 3.

I considered putting this at the very top of the document, but I spotted
this section that seemed to fit.

Addresses airbnb#1057

Author: lencioni

README.md

@@ -2772,6 +2772,11 @@ Other Style Guides
 1. [Iterators and Generators](#iterators-and-generators)
 1. [Modules](#modules)
 
+  <a name="tc39-proposals"></a>
+  - [27.2](#tc39-proposals) Do not use [TC39 proposals](https://github.com/tc39/proposals) that have not reached stage 3.
+
+    > Why? [They are not finalized](https://tc39.github.io/process-document/), and they are subject to change or to be withdrawn entirely. We want to use JavaScript, and proposals are not JavaScript yet.
+
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 ## Testing