Salvage updates from src:tails-installer's debian/README.source (refs: #12152)

That file was deleted to avoid duplication but some of its improvements
were lost in the process.

wiki/src/contribute/release_process/tails-installer.mdwn

@@ -268,12 +268,12 @@ package in sid accordingly, for example:
 
 Example for a backport to Jessie:
 
-* check out the `debian/jessie-backports` branch
+* check out the `debian/stretch-backports` branch
 * merge the `debian/sid` branch
-* `dch --bpo` to bump version to `5.3+dfsg-1~bpo8+1`
-* build, test and upload to jessie-backports
-* have gbp create a `debian/5.3+dfsg-1_bpo8+1` tag
-* push the Debian packaging branch (`debian/jessie-backports`) and the new tag
+* `dch --bpo` to bump version to `5.3+dfsg-1~bpo9+1`
+* build, test and upload to stretch-backports
+* have gbp create a `debian/5.3+dfsg-1_bpo9+1` tag
+* push the Debian packaging branch (`debian/stretch-backports`) and the new tag
 
 Upload a package to our Ubuntu PPA
 ----------------------------------