Release process: workaround po4a changes by requiring the version from Stretch for now.

wiki/src/contribute/release_process.mdwn

@@ -21,6 +21,8 @@ To release Tails you'll need some packages installed:
   `debian/control` in the `debian` branch of its repo)
 * `tails-perl5lib` dependencies (same trick as `tails-iuk` to get the
   list)
+* `po4a` _from Stretch_: the version in testing/sid extracts Markdown headings
+   in a different way, which makes tons of strings fuzzy.
 
 Environment
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