Implement Org linting

* lisp/org-lint.el: New file.

* doc/org.texi (Org syntax):
* etc/ORG-NEWS:  Document new feature.

doc/org.texi

@@ -2075,6 +2075,10 @@ abstract structure.  The export engine relies on the information stored in
 this list.  Most interactive commands (e.g., for structure editing) also
 rely on the syntactic meaning of the surrounding context.
 
+@cindex syntax checker
+@cindex linter
+You can check syntax in your documents using @code{org-lint} command.
+
 @node Tables
 @chapter Tables
 @cindex tables

etc/ORG-NEWS

@@ -9,6 +9,9 @@ See the end of the file for license conditions.
 Please send Org bug reports to emacs-orgmode@gnu.org.
 
 * Version 8.4
+** New features
+*** Org linter
+~org-lint~ can check syntax and report common issues in Org documents.
 ** New functions
 ~org-show-children~ is a faster implementation of
 ~outline-show-children~.