Add note on using curl to test /encrypt and /decrypt

Fixes gh-246

docs/src/main/asciidoc/spring-cloud-config.adoc

@@ -404,6 +404,11 @@ $ curl localhost:8888/decrypt -d 682bc583f4641835fa2db009355293665d2647dade3375c
 mysecret
 ----
 
+TIP: If you are testing like this with curl, then use
+`--data-urlencode` (instead of `-d`) or set an explicit `Content-Type:
+text/plain` to make sure curl encodes the data correctly when there
+are special characters ('+' is particularly tricky).
+
 Take the encrypted value and add the `{cipher}` prefix before you put
 it in the YAML or properties file, and before you commit and push it
 to a remote, potentially insecure store. The `/encrypt` and `/decrypt`