Indicate that some distributions require building from sources

Author: mgorny

source/discussions/downstream-packaging.rst

@@ -54,9 +54,10 @@ Why?
 ~~~~
 
 The vast majority of downstream packagers prefer to build packages from source,
-rather than use the upstream-provided binary packages. This is also true
-of pure Python packages that provide universal wheels. The reasons for using
-source distributions may include:
+rather than use the upstream-provided binary packages. In some cases, using
+sources is actually required for the package to be included in the distribution.
+This is also true of pure Python packages that provide universal wheels.
+The reasons for using source distributions may include:
 
 - Being able to audit the source code of all packages.