diff --git a/docs/cheatsheet.md b/docs/cheatsheet.md
index b7b3855..03c1de7 100644
--- a/docs/cheatsheet.md
+++ b/docs/cheatsheet.md
@@ -18,3 +18,8 @@ Print content of a TLS certificate:
 
     openssl x509 -in certs/xmpp.pem -text
 
+Make sure `xmpp.key` starts and ends with:
+```
+-----BEGIN RSA PRIVATE KEY-----
+-----END RSA PRIVATE KEY-----
+```