Add to beginning of `auto-mode-alist', not end

In Emacs 26, a file called 'CHANGELOG.md' will open in
`changelog-mode' instead of `markdown-mode', because a regexp was
added to `auto-mode-alist' that matches the 'CHANGELOG' part of the
filename. In order for the file to correctly open in `markdown-mode',
the regexps for `markdown-mode' must be added earlier in
`auto-mode-alist', hence this commit.

There should be no concern about this overriding user configuration,
since it makes no sense for user configuration code to be run before
the autoloads for `markdown-mode' are evaluated.

CHANGES.md

@@ -15,6 +15,12 @@
         `markdown-font-lock-keywords` is now used instead, but users
         should use `font-lock-add-keywords` instead of modifying this
         variable.
+    -   `markdown-mode` now adds entries to the beginning of
+        `auto-mode-alist` rather than the end. If you were relying on
+        the previous behavior in order to override these entries, you
+        should fix the problem by following best practice and ensuring
+        that your user configuration is loaded after the autoloads for
+        `markdown-mode` are evaluated.
 
 *   New features:
 

markdown-mode.el

@@ -9525,9 +9525,9 @@ spaces, or alternatively a TAB should be used as the separator."
   (add-hook 'kill-buffer-hook #'markdown-live-preview-remove-on-kill t t))
 
 ;;;###autoload
-(add-to-list 'auto-mode-alist '("\\.markdown\\'" . markdown-mode) t)
+(add-to-list 'auto-mode-alist '("\\.markdown\\'" . markdown-mode))
 ;;;###autoload
-(add-to-list 'auto-mode-alist '("\\.md\\'" . markdown-mode) t)
+(add-to-list 'auto-mode-alist '("\\.md\\'" . markdown-mode))
 
 
 ;;; GitHub Flavored Markdown Mode  ============================================