transitions : fill the section with contents from the ubuntu maintainer handbook

Author: hector-cao

docs/how-ubuntu-is-made/processes/transitions.md

@@ -1,2 +1,21 @@
 (transitions-process)=
-# How transitions work
+
+# Library Transitions
+
+A library transition is the process that happens when a library gets updated to a new
+version that includes API/ABI changes and requires rebuilding and re-testing all
+the reverse dependencies. A library transition can be more or less complex depending
+on the API/ABI breaking change and the number of reverse dependencies, especially
+when some of them fail to build with the new package version.
+
+Very complex transitions require a concerted, organized effort, and that's what we'll
+primarily focus on in this document. Simpler transitions often go through automatically,
+or sometimes with a bit of manual assistance.
+
+To keep track of all recent, planned or ongoing transitions, there is the Launchpad project
+[Ubuntu Transition Tracker](https://launchpad.net/ubuntu-transition-trackeer) managed
+by the [Ubuntu Transition Trackers](https://launchpad.net/ubuntu-transition-tracker) team.
+
+There is a page [NBS](https://ubuntu-archive-team.ubuntu.com/nbs.html) that lists all binary
+packages in the archive (and its reverse dependencies) that are not built from any source
+package. These packages existence in the list might result from an ongoing transition.