[enh] Improve installation documentation for Debian Jessie.

README.md

@@ -12,6 +12,10 @@ Third-party dependencies:
  * ``sqlite``
  * ``libspatialite``
 
+On a Debian Jessie system, do:
+
+    apt install sqlite3 libspatialite-dev
+
 Preferably in a virtualenv, run:
 
     pip install -r requirements.txt