Update readme instructions for adding overlay

README.md

@@ -24,6 +24,20 @@ Using eselect-repository:
 # eselect repository add projectmoon git https://git.agnos.is/projectmoon/projectmoon-overlay
 ```
 
+It usually a best practice to mask packages from overlays, except the ones you want:
+
+In `/etc/portage/package.mask`:
+```
+*/*::projectmoon
+```
+
+Individual packages can be unmasked (and added to
+`package.accept_keywords` if necessary). In `/etc/portage/package.unmask`:
+```
+sys-kernel/libre-kernel::projectmoon
+virtual/dist-kernel::projectmoon
+```
+
 ## The Linux-libre dist-kernel
 
 This overlay provides a dist-kernel (based on