contributor docs: Add section on machine translation in Transifex.

docs/translating/translating.md

@@ -157,6 +157,26 @@ There are a few ways to see your translations in the Zulip UI:
 
   This can occasionally be useful for debugging.
 
+### Machine translation
+
+Transifex has [built-in machine translation
+capabilities](https://help.transifex.com/en/articles/6271130-setting-up-machine-translation).
+If machine translation is enabled for your language, you can generate one by
+clicking the **lightning** symbol above the translation box.
+
+![Transifex machine translation button](../images/transifex-machine-translation-button.png)
+
+There are a few things to keep in mind if you choose to make use of machine
+translation:
+
+- We expect human-quality translations for Zulip. While machine translation can
+  be a helpful aid, please be sure to review all machine translated strings.
+
+- We are using machine translation in HTML mode, which has a known bug that
+  causes special characters to be rendered as their HTML encodings. For example,
+  a `'` will be converted to ` '`. Please be sure to fix this when reviewing
+  translations.
+
 ### Translation style guides
 
 We maintain translation style guides for Zulip, giving guidance on how