Add FAQ section on stability guarantees.

content/docs/introduction/faq.md

@@ -30,6 +30,17 @@ The Alertmanager cannot currently be made highly available, but this is a goal.
 
 Most Prometheus components are written in Go. Some are also written in Java and Ruby.
 
+### How stable are Prometheus features, storage formats, and APIs?
+
+Although Prometheus and many of its ecosystem components are already quite
+stable, we will still allow for occasional breaking changes until the
+Prometheus server reaches version 1.0.0. These breaking changes will be pointed
+out in release announcements for components that already have a proper release
+process (like the Prometheus server) or communicated clearly otherwise. After
+releasing version 1.0.0, breaking changes will be indicated by increments of
+the major version. See also the documentation for [semantic
+versioning](http://semver.org/), which we are following.
+
 ### Why do you pull rather than push?
 
 Pulling over HTTP offers a number of advantages: