Clean up .gitignore and make a note about using global ignores

.gitignore

@@ -1,26 +1,23 @@
-pkg
-.bundle
-Gemfile.lock
+# Don't put *.swp, *.bak, etc here; those belong in a global ~/.gitignore.
+# Check out http://help.github.com/ignore-files/ for how to set that up.
+
 debug.log
-doc/rdoc
-activemodel/doc
-activeresource/doc
-activerecord/doc
-activerecord/sqlnet.log
-actionpack/doc
-actionmailer/doc
-activesupport/doc
-activesupport/test/tmp
-activemodel/test/fixtures/fixture_database.sqlite3
-actionpack/test/tmp
-activesupport/test/fixtures/isolation_test
-dist
-railties/test/500.html
-railties/test/fixtures/tmp
-railties/test/initializer/root/log
-railties/doc
-railties/guides/output
-railties/tmp
-.rvmrc
-.rbenv-version
-RDOC_MAIN.rdoc
+/.bundle
+/.rbenv-version
+/.rvmrc
+/Gemfile.lock
+/pkg
+/dist
+/doc/rdoc
+/*/doc
+/*/test/tmp
+/activerecord/sqlnet.log
+/activemodel/test/fixtures/fixture_database.sqlite3
+/activesupport/test/fixtures/isolation_test
+/railties/test/500.html
+/railties/test/fixtures/tmp
+/railties/test/initializer/root/log
+/railties/doc
+/railties/guides/output
+/railties/tmp
+/RDOC_MAIN.rdoc