doc: add package name restrictions

doc/files/package.json.md

@@ -17,14 +17,23 @@ them.  The name and version together form an identifier that is assumed
 to be completely unique.  Changes to the package should come along with
 changes to the version.
 
-The name is what your thing is called.  Some tips:
+The name is what your thing is called.
 
+Some rules:
+
+* The name must be shorter than 214 characters. This includes the scope for
+  scoped packages.
+* The name can't start with a dot or an underscore.
+* New packages must not have uppercase letters in the name.
+* The name ends up being part of a URL, an argument on the command line, and a
+  folder name. Therefore, the name can't contain any non-URL-safe characters.
+
+Some tips:
+
+* Don't use the same name as a core Node module.
 * Don't put "js" or "node" in the name.  It's assumed that it's js, since you're
   writing a package.json file, and you can specify the engine using the "engines"
   field.  (See below.)
-* The name ends up being part of a URL, an argument on the command line, and a
-  folder name. Any name with non-url-safe characters will be rejected.
-  Also, it can't start with a dot or an underscore.
 * The name will probably be passed as an argument to require(), so it should
   be something short, but also reasonably descriptive.
 * You may want to check the npm registry to see if there's something by that name