Add news entry

news/pf_strings.rst

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+**Added:**
+
+* New xonsh syntax ``pf`` strings -- combining path strings with f-strings.
+
+  Usage:
+
+  .. code-block:: bash
+       gil@bad_cat ~ $ repos = 'github.com'
+       gil@bad_cat ~ $ pf"~/{repos}"
+       PosixPath('/home/gil/github.com')
+       gil@bad_cat ~ $ pf"{$HOME}"
+       PosixPath('/home/gil')
+       gil@bad_cat ~ $ pf"/home/${'US' + 'ER'}"
+       PosixPath('/home/gil')
+
+**Changed:**
+
+* <news item>
+
+**Deprecated:**
+
+* <news item>
+
+**Removed:**
+
+* <news item>
+
+**Fixed:**
+
+* <news item>
+
+**Security:**
+
+* <news item>