Added clarification that the plink is needed

Most users don't have plink installed by default, and it is not mentioned in any previous steps.  Would speed up troubleshooting if it is mentioned as a requirement

README.md

@@ -1319,7 +1319,7 @@ Run the following commands:
 You can then change your repository url to:
 `git@github.com:USERNAME/repository`. Any authenticated commands will be authorized by your YubiKey.
 
-**Note:** If you encounter the error `gpg: signing failed: No secret key`, run `gpg --card-status` with your YubiKey plugged in and try the git command again.
+**Note:** If you encounter the error `gpg: signing failed: No secret key`, run `gpg --card-status` with your YubiKey plugged in and try the git command again.  If you don't have it already, install [plink](https://linux.die.net/man/1/plink) using `sudo apt-get install plink`
 
 ## 4.7 Requiring touch to authenticate