README: add note about advanced options (#218)

* :memo: Add note about advaced options in README

Clarify position of advanced options argumnts (they won’t work if are placed after domain names)

Add example.

* Update README.md

README.md

@@ -144,6 +144,14 @@ To only install the local root CA into a subset of them, you can set the `TRUST_
 	    all other flags and arguments except -install and -cert-file.
 ```
 
+> **Note:** You _must_ place these options before the domain names list.
+
+#### Example
+
+```
+mkcert -key-file key.pem -cert-file cert.pem example.com *.example.com
+```
+
 ### S/MIME
 
 mkcert automatically generates an S/MIME certificate if one of the supplied names is an email address.