Disable HHAST by default

it executes $PROJECT/vendor/bin/hhast, so that's not a good idea.

doc/ale-hack.txt

@@ -1,5 +1,14 @@
 ===============================================================================
 ALE Hack Integration                                         *ale-hack-options*
+                                                         *ale-integration-hack*
+
+  HHAST is disabled by default, as it executes code in the project root.
+
+  Currently linters must be enabled globally.  HHAST can be enabled with:
+
+>
+  let g:ale_linters = {'hack': ['hack', 'hhast']}
+<
 
 ===============================================================================
 hack                                                            *ale-hack-hack*

doc/ale.txt

@@ -1214,6 +1214,7 @@ g:ale_linters                                                   *g:ale_linters*
   {
   \   'csh': ['shell'],
   \   'go': ['gofmt', 'golint', 'go vet'],
+  \   'hack': ['hack'],
   \   'help': [],
   \   'perl': ['perlcritic'],
   \   'python': ['flake8', 'mypy', 'pylint'],