Add privacy policy to the git repo

It was previously hosted at [1] alongside the privacy policies for other
extensions.

[1]: https://labs.avm99963.com/chrome/gtranslator_extension/cws_privacy.html

Change-Id: Ibc45ea33974da7edf8b2fe5a2be3747fe69475a1

docs/privacy_policy.md

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+# Privacy policy for the Translate Selected Text extension
+
+The Translate Selected Text extension allows you to translate a piece of text
+that you have selected, by right clicking the selected text and clicking the
+context menu extension option. When doing this, the text you selected will be
+used for the sole purpose of opening a new Google Translate tab/window which
+will show you the translation, but the extension will not keep any record of
+that nor share it with any third party. The use of Google Translate is subject
+to their [Terms of Service and Privacy Policy][1].
+
+Your extension options will be saved in your browser, and synced between all the
+browsers you've synced in the browser settings.
+
+## Contact information
+
+You can contact us here:
+
+* **Name:** Adrià Vilanova Martínez
+* **E-mail address:** me+privacypolicy _[at]_ avm99963.com
+
+[1]: https://policies.google.com/