sbcl: add update notice for maxima

Closes Homebrew/homebrew#17218.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

Formula/sbcl.rb

@@ -77,4 +77,12 @@ class Sbcl < Formula
     ENV['INSTALL_ROOT'] = prefix
     system "sh install.sh"
   end
+
+  def caveats; <<-EOS.undent
+    If you are upgrading sbcl and you have installed maxima,
+    you have to reinstall maxima:
+
+      brew rm maxima && brew install maxima
+    EOS
+  end
 end