Add explicit MPL-2.0 licence notice "Exhibit A"

Unlike most other Free Software licences, the MPL explicitly says it
applies only to files with a specific notice:

    1.4. "Covered Software"
	means Source Code Form to which the initial Contributor has attached
	the notice in Exhibit A, the Executable Form of such Source Code
	Form, and Modifications of such Source Code Form, in each case
	including portions thereof.

Unfortunately, this means that copying the MPL text into tree, and
setting the "license" Cargo option, leaves an ambiguous situation.
One might presume that the intent was to actually *use* the MPL for
the whole project, but the legal licence text explicitly rejects that.

Thankfully, according to the licence text, it is not actually
necessary to add the notice to *every* file:

    If it is not possible or desirable to put the notice in a particular
    file, then You may include the notice in a location (such as a LICENSE
    file in a relevant directory) where a recipient would be likely to look
    for such a notice.

So clarify this situation by adding two centrals explicit copy of the
MPL "Exhibit A" text, one in each affected crate subtree, and declare
them to apply to everything.

Putting these in Cargo.toml puts them next to the "license =" tag.
Another possibility would be `README.md` but IMO this legal
technicality doesn't really warrant such exposure.

(Putting it in the LICENCE file, as the text itself suggests, would
mean either (i) renaming the verbatim copy of the MPL 2.0 and writing
a new file or (ii) adding it as a rubric to the top of the MPL 2.0
text in LICENCE - resulting in a LICENCE file which is not identical
to the usual MPL-2.0 text file.  Neither of those seem desirable.

Putting just once it at the repository toplevel would mean it didn't
appear in .crate files, and also require explicitly stating which
subsections of the repository are covered.)

webpki-root-certs/Cargo.toml

@@ -8,6 +8,15 @@ homepage.workspace = true
 repository.workspace = true
 description = "Mozilla trusted certificate authorities in self-signed X.509 format for use with crates other than webpki"
 
+# The following notice applies to all the files in this source subtree:
+#
+#     This Source Code Form is subject to the terms of the Mozilla Public
+#     License, v. 2.0. If a copy of the MPL was not distributed with this
+#     file, You can obtain one at http://mozilla.org/MPL/2.0/.
+#
+# (If you copy files from here into another program which is not
+# licenced MPL-2.0, you should add that notice to each copied file.)
+
 [dependencies]
 pki-types = { workspace = true }
 

webpki-roots/Cargo.toml

@@ -8,6 +8,15 @@ homepage = { workspace = true }
 repository = { workspace = true }
 description = "Mozilla's CA root certificates for use with webpki"
 
+# The following notice applies to all the files in this source subtree:
+#
+#     This Source Code Form is subject to the terms of the Mozilla Public
+#     License, v. 2.0. If a copy of the MPL was not distributed with this
+#     file, You can obtain one at http://mozilla.org/MPL/2.0/.
+#
+# (If you copy files from here into another program which is not
+# licenced MPL-2.0, you should add that notice to each copied file.)
+
 [dependencies]
 pki-types = { workspace = true }