Commit 49e5d9a2db5b3365dd5af683ee519e078d8a83e7

Add some details on getting the user URI (required) and profile URL (recommended).
Tweak notes to clarify PuSH hub link is required in feed.
Note that external profile info sources are acceptable but not required.
  
225225 actor. In this case, the author element MUST have a unique
226226 identifier, that is, either the <email> or <uri<
227227 elements must be present.</t>
228 <t>Subscribers MUST search for the URI in the following order:
229 the Activity object's &lt;id&gt; element if available, else
230 the Atom author's &lt;uri&gt; element if available, else the
231 Atom author's &lt;email&gt; element treated as a Webfinger acct: URI.
232 Publishers MAY use a profile URL as the URI, but if so
233 MUST ensure that they are stable permalinks.</t>
234 <t>Users SHOULD have a profile URL, which SHOULD be an HTTP
235 or HTTPS reference to an HTML page including discovery
236 information for the user's feed.
237 The profile URL SHOULD be represented as a
238 link[@rel=alternate,@type=text/html] in the Activity subject,
239 actor, or object item, otherwise the URI MAY be used if it is
240 an HTTP or HTTPS URL.</t>
228241 <t>To provide additional profile information, the author or
229242 actor element can be extended with optional <xref
230243 target="poco">Portable Contacts</xref>-structured data. Some
255255 activity actors.</t>
256256 </list>
257257 </t>
258 <t>Subscribers MAY use other sources of profile information such
259 as microformats on the profile page, but are not required to.</t>
258260 </section>
259261 <section title="Feed representation">
260262 <t>Feeds are Activity feeds per <xref target="activitiesinatom" />.</t>
264264 User feeds SHOULD have a single activity:subject or atom:author.
265265 Group feeds SHOULD have a single activity:subject
266266 representing the group.</t>
267 <t>All feeds SHOULD have a link[@rel=hub] and a
268 link[@rel=salmon] to identify the</t>
267 <t>All feeds MUST have a link[@rel=hub] to identify a PubSubHubbub
268 endpoint URL to establish update subscriptions.</t>
269 <t>All feeds SHOULD have a link[@rel=salmon] to identify the Salmon
270 endpoint URL to receive replies and notifications.</t>
269271 </section>
270272 <section title="Update publication">
271273 <t>The publisher server uses <xref