Add the description of hacking rule T107

HACKING.rst doesn't contain T107 description, so this patch adds it.

Change-Id: I2e9a0c190016c3007ab37b502013bff222a9fc3d
diff --git a/HACKING.rst b/HACKING.rst
index 607682b..81a7c2c 100644
--- a/HACKING.rst
+++ b/HACKING.rst
@@ -13,6 +13,7 @@
 - [T104] Scenario tests require a services decorator
 - [T105] Tests cannot use setUpClass/tearDownClass
 - [T106] vim configuration should not be kept in source files.
+- [T107] Check that a service tag isn't in the module path
 - [N322] Method's default argument shouldn't be mutable
 
 Test Data/Configuration