Require a short help string for "ls"

The linter change will enforce it for scripts.  Plugins always include
the line, and length will have to be checked manually.

docs/Contributing.rst

@@ -137,6 +137,11 @@ there are a few important standards for completeness and consistent style.  Trea
 this section as a guide rather than iron law, match the surrounding text, and you'll
 be fine.
 
+Each command should have a short (~54 character) help string, which is shown
+by the `ls` command.  For scripts, this is a comment on the first line
+(the comment marker and whitespace is stripped).  For plugins it's the second
+argument to ``PluginCommand``.  Please make this brief but descriptive!
+
 Everything should be documented!  If it's not clear *where* a particular
 thing should be documented, ask on IRC or in the DFHack thread on Bay12 -
 as well as getting help, you'll be providing valuable feedback that

travis/script-in-readme.py

@@ -13,12 +13,25 @@ def expected_cmd(path):
     return fname
 
 
+def check_ls(fname, line):
+    """Check length & existence of leading comment for "ls" builtin command."""
+    line = line.strip()
+    comment = '--' if fname.endswith('.lua') else '#'
+    if line.endswith('=begin') or not line.startswith(comment):
+        print('Error: no leading comment in ' + fname)
+        return 1
+    if len(line.replace(comment, '').strip()) > 53:
+        print('Error: leading comment too long in ' + fname)
+        return 1
+    return 0
+
+
 def check_file(fname):
     errors, doclines = 0, []
     with open(fname, errors='ignore') as f:
-        for l in f.readlines():
-            if not l.strip():
-                continue
+        lines = f.readlines()
+        errors += check_ls(fname, lines[0])
+        for l in lines:
             if doclines or l.strip().endswith('=begin'):
                 doclines.append(l.rstrip())
             if l.startswith('=end'):