Add Payload-Files-Required and Payload-Files-Allowed (#42)

* Clarify MUST/SHOULD for Tag-Files-*

See discussion in #36

* Add Payload-Files-Required and Payload-Files-Allowed

This fixes #36

* fix grammar mistakes

Credit to @ruebot

index.html

@@ -250,10 +250,10 @@ <h2>Implementation Details</h2>
           <li>
             `Tag-Files-Required`: LIST
             <p>
-              A list of a tag files that must be included in a conformant Bag. Entries are full path names relative to the Bag base directory. As per [[RFC8493]] <a href="https://tools.ietf.org/html/rfc8493#section-2.2.4">section 2.2.4</a>, these tag files need not be listed in tag manifiest files. `Tag-Files-Required` SHOULD NOT include `bag-info.txt` (which is always required), nor any required manifest files, which instead are required by `Manifests-Required` and `Tag-Manifests-Required`.
+              A list of a tag files that MUST be included in a conformant Bag. Entries are full path names relative to the Bag base directory. As per [[RFC8493]] <a href="https://tools.ietf.org/html/rfc8493#section-2.2.4">section 2.2.4</a>, these tag files need not be listed in tag manifiest files. `Tag-Files-Required` SHOULD NOT include `bag-info.txt` (which is always required), nor any required manifest files, which instead are required by `Manifests-Required` and `Tag-Manifests-Required`.
             </p>
             <p>
-              Every file in `Tag-Files-Required` must also be present in `Tag-Files-Allowed`.
+              Every file in `Tag-Files-Required` MUST also be covered by the path patterns in `Tag-Files-Allowed`.
             </p>
           </li>
           <li>
@@ -262,13 +262,34 @@ <h2>Implementation Details</h2>
               A list of tag files that MAY be included in a conformant Bag. Entries are either full path names relative to the bag base directory or path name patterns in which asterisks can represent zero or more characters (c.f. <a href="http://man7.org/linux/man-pages/man7/glob.7.html">glob(7)</a>).
             </p>
             <p>
-              If `Tag-Files-Allowed` is not provided, its value is assumed to be `['*']`, i.e. all tag files are allowed.
+              Conformants bags MUST NOT contain any payload that does not match the specified `Tag-Files-Allowed`. If `Tag-Files-Allowed` is not provided, its value is assumed to be `['*']`, i.e. all tag files are allowed.
             </p>
             <p>
               As per [[RFC8493]] <a href="https://tools.ietf.org/html/rfc8493#section-2">section 2</a>, these tag files need not be listed in tag manifest files. `Tag-Files-Required` SHOULD NOT include `bag-info.txt` (which is always required), nor any required manifest files, which instead are required by `Manifests-Required` and `Tag-Manifests-Required`.
             </p>
             <p>
-              At least all the tag files listed in `Tag-Files-Required` must be in included in `Tag-Files-Allowed`.
+              At least all the tag files listed in `Tag-Files-Required` MUST be in included in `Tag-Files-Allowed`.
+            </p>
+          </li>
+          <li>
+            `Payload-Files-Required`: LIST
+            <p>
+              A list of a payload files and/or directories that MUST be included in a conformant Bag. Entries are full path names relative to the Bag base directory, e.g. `data/LICENSE.txt` or `data/src/`.  Paths requiring directory existence MUST end with `/`, indicating that in conformant Bags, the directory MUST be present with at least one file or subdirectory (which may have a placeholder file to indicate an empty directory, see [[RFC8493]] <a href="https://tools.ietf.org/html/rfc8493#section-2.1.3">section 2.1.3</a>).
+            </p>
+            <p>
+              Every file in `Payload-Files-Required` MUST also be covered by the path patterns in `Payload-Files-Allowed`.
+            </p>
+          </li>
+          <li>
+            `Payload-Files-Allowed`: LIST
+            <p>
+              A list of payload files or directories that MAY be included in a conformant Bag. Each entry MUST be either a full path name relative to the bag base directory, or a path name pattern in which asterisks can represent zero or more characters (c.f. [glob(7)](http://man7.org/linux/man-pages/man7/glob.7.html)). Paths requiring permitted directories MUST end with `/*` (not `/`).<br>
+            </p>
+            <p>
+              Conformants bags MUST NOT contain any payload that does not match the specified `Payload-Files-Allowed`. If `Payload-Files-Allowed` is not provided, its value is assumed to be ['*'], i.e. all payload files and directories are allowed.
+            </p>
+            <p>
+              At least all the payload paths listed in `Payload-Files-Required` MUST be covered by the list of path patterns in `Payload-Files-Allowed`.
             </p>
           </li>
         </ol>