document the .gpg extension requirement

diff --git a/README b/README
index bcec047adec3c906a7043c7aa134f9adfa8fee08..85cf6df7d181f0678452d0f94e223f1a86c91161 100644 (file)
--- a/README
+++ b/README
@@ -495,6 +495,8 @@ is assumed by secure apt to be binary OpenPGP keys and *not*
 "ascii-armored" or "plain text" OpenPGP key material. For the latter,
 use `apt::key::plain`.
 
+The `.gpg` extension is compulsory for `apt` to pickup the key properly.
+
 apt::key::plain
 ---------------