phrase locale names

- added important bit about naming your locales in phrase as the
  used upload format (symfony xliff) causes phrase to ignore the
  locale_id send with the uploads and uses the extracted value
  from the target-language attribute in the xliff file for matching

Author: wickedOne

translation.rst

@@ -698,6 +698,9 @@ configure the ``providers`` option:
     If you use Phrase as a provider you must configure a user agent in your dsn. See
     `Identification via User-Agent`_ for reasoning and some examples.
 
+    Also make the locale _names_ in Phrase should be as defined in RFC4646 (e.g. pt-BR rather than pt_BR).
+    Not doing so will result in Phrase creating a new locale for the imported keys.
+
 .. tip::
 
     If you use Lokalise as a provider and a locale format following the `ISO