explain how to install diffoscope on Gentoo

Signed-off-by: Eli Schwartz <eschwartz93@gmail.com>

index.md

@@ -72,6 +72,9 @@ You can try it now using Docker:
 * On <a href="https://fedoraproject.org">Fedora</a> based systems:<br/>
 `dnf install diffoscope`
 
+* On <a href="https://www.gentoo.org">Gentoo</a> based systems:<br/>
+`emerge dev-util/diffoscope`
+
 * On <a href="https://opensuse.org">openSUSE</a>:<br/>
 `zypper in diffoscope`