Add a document that describes profiling eventlet services

Profiling eventlet using services is a bit different from standard
situations so here is a document that tries to explain the basics
on how to get started doing it.

Change-Id: If8c34653285f07c5cc1abccabfec16f18daafdde

doc/source/index.rst

@@ -217,6 +217,7 @@ looking parts of our architecture. These are collected below.
    contributor/testing/serial-console
    contributor/testing/zero-downtime-upgrade
    contributor/testing/down-cell
+   contributor/testing/eventlet-profiling
    contributor/how-to-get-involved
    contributor/process
    contributor/project-scope