Add NOTE clarifying presentation URL scheme support

[markafoltz]

Updates spec for proposed resolution of Issue #76.

[tidoust]

The sentence "User agents must accept a presentationUrl using the http or https schemes" sounds very normative to me, which looks strange in a note that is informative-only by definition.

I suggest to rewrite to:

<p class="note">The <code>presentationUrl</code> should name a resource accessible
to the local or a remote user agent. This specification defines behavior for
<code>presentationUrl</code> using the <code>http</code> or <code>https</code>
schemes; behavior for other schemes is not defined by this specification.</p>

Note that user agents will de facto be required to support http and https schemes because algorithms will mandate behavior in such cases. As such, there should be no need to add an extra normative statement such as "user agents MUST support presentationUrl using the http or https schemes".

The "should" in "The presentationUrl should name a resource accessible" could also be read as normative but it does not apply to user agents, so less of a problem.

[louaybassbouss]

@mfoltzgoogle the suggestion from @tidoust LGTM

[markafoltz]

Thanks @tidoust and @louaybassbouss. I've updated the PR with the suggested text and will now merge.