docs: Clarify that pinentry needs to point to a GUI program

As seen in #3 the GPG installation from homebrew creates a `pinentry`
symlink pointing the `pinentry-curses` which prevents `pinentry-touchid`
from successfully calling the fallback pinentry program
(`pinentry-mac`).

README.md

@@ -37,6 +37,16 @@ This program interacts with the `gpg-agent` for providing a password, using the
 
 ### Homebrew
 
+If you have already installed GPG, make sure that executing `pinentry` shows a GUI prompt by running
+the following command:
+
+```
+echo GETPIN | pinentry
+```
+
+You should get the dialog from `pinentry-mac`. If that is not the case you can overwrite the
+`pinentry` symlink to point to `pinentry-mac`.
+
 As part of our release process we keep an updated Homebrew Formula. To install pinentry-touchid using
 homebrew execute the following commands: