Comment in requirements.txt

Author: spookylukey

requirements.txt

@@ -1,3 +1,8 @@
+# This file is used in test runs, to pin versions of runtime dependencies
+# (those in setup.cfg), to help test stability, because some tests
+# depend on exact behaviour (e.g. of Babel which bundles specific versions
+# of i18n data) and are hard to write concisely otherwise.
+
 fluent.syntax>=0.14
 attrs==19.3.0
 babel==2.9.1