Add Keg for building

Keg is a candidate to replace Cask in the future, but Cask is not yet
removed.

https://github.com/conao3/keg.el

Author: zonuexe

Diff for: .gitignore

@@ -1,4 +1,5 @@
 /.cask/
+/.keg/
 /.mailmap
 /php-mode-autoloads.el
 *~

Diff for: Keg

@@ -0,0 +1,10 @@
+(source gnu melpa)
+
+(package
+ (php-mode
+  (recipe . (php-mode :fetcher github :repo "emacs-php/php-mode.el"
+                      :files (:defaults "lisp/*.el")))))
+
+(dev-dependency pkg-info)
+(dev-dependency projectile)
+(dev-dependency shut-up)