Document installing borgmatic with pip install --user instead of a system Python install.

NEWS

@@ -1,3 +1,6 @@
+1.3.4.dev0
+ * Document installing borgmatic with pip install --user instead of a system Python install.
+
 1.3.3
  * Add validate-borgmatic-config command, useful for validating borgmatic config generated by
    configuration management or even edited by hand.