Automatic caveats for emacs lisp files

- The name of the subdirectory under site-lisp may not be exactly the
  same as the name of the package

- `(normal-top-level-add-subdirs-to-load-path)` adds every subdirectory
  of `default-directory` to the load-path, so users don't have to update
  their load-path for every package they install.

Closes Homebrew/homebrew#42309.

Library/Homebrew/caveats.rb

@@ -16,6 +16,7 @@ class Caveats
     caveats << plist_caveats
     caveats << python_caveats
     caveats << app_caveats
+    caveats << elisp_caveats
     caveats.compact.join("\n")
   end
 
@@ -132,6 +133,19 @@ class Caveats
     end
   end
 
+  def elisp_caveats
+    if keg && keg.elisp_installed?
+      <<-EOS.undent
+        Emacs Lisp files have been installed to:
+        #{HOMEBREW_PREFIX}/share/emacs/site-lisp/
+
+        Add the following to your init file to have packages installed by Homebrew added to your load-path:
+        (let ((default-directory "#{HOMEBREW_PREFIX}/share/emacs/site-lisp/"))
+          (normal-top-level-add-subdirs-to-load-path))
+      EOS
+    end
+  end
+
   def plist_caveats
     s = []
     if f.plist or (keg and keg.plist_installed?)

Library/Homebrew/keg.rb

@@ -243,6 +243,10 @@ class Keg
     Dir["#{path}/{,libexec/}*.app"].any?
   end
 
+  def elisp_installed?
+    Dir["#{path}/share/emacs/site-lisp/**/*.el"].any?
+  end
+
   def version
     require 'pkg_version'
     PkgVersion.parse(path.basename.to_s)