certfp.md: Add mIRC (#389)

Author: RealKindOne

content/_guides/certfp.md

@@ -181,6 +181,18 @@ right of the screen to save.
    respectively.
 9. Click `OK` or `Apply`.
 
+### mIRC
+
+1. In the File menu, click Select Servers...
+2. In the Connect -> Servers section of the mIRC Options window,
+   select Libera.Chat, click the hamburger menu button, then click Edit.
+3. In the `Login Method` dropdown, select `SASL External /CAP`.
+4. Click the `SSL` Tab.
+5. Check `Use private certificate`.
+6. Click the rectange box below and select the certificate file.
+   The file must contain both the certificate and private key.
+7. Click the OK button.
+
 ## Add your fingerprint to NickServ
 
 You can then check whether you have a fingerprint by using `whois` on yourself: