mail-filter/spamassassin: update metadata.xml with new maintainers.

We had a few users request to proxy-maintain this package. Now that
its open bugs are fixed, this commit adds two of those users (Philippe
Chaintreuil and Marcin Mirosław) to metadata.xml. The Proxy
Maintainers project has been added to support them.

I've added myself, too, for now, so that I will be CCed on bugs
arising from my recent changes.

Gentoo-Bug: 583908

Package-Manager: portage-2.2.28

mail-filter/spamassassin/metadata.xml

@@ -1,7 +1,26 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <!DOCTYPE pkgmetadata SYSTEM "http://www.gentoo.org/dtd/metadata.dtd">
 <pkgmetadata>
-  <!-- maintainer-needed -->
+  <maintainer type="person">
+    <email>[email protected]</email>
+    <name>Philippe Chaintreuil</name>
+  </maintainer>
+
+  <maintainer type="person">
+    <email>[email protected]</email>
+    <name>Marcin Mirosław</name>
+  </maintainer>
+
+  <maintainer type="person">
+    <email>[email protected]</email>
+    <name>Michael Orlitzky</name>
+  </maintainer>
+
+  <maintainer type="project">
+    <email>[email protected]</email>
+    <name>Proxy Maintainers</name>
+  </maintainer>
+
   <use>
     <flag name="bayes">
       Require a database (MySQL, SQLite, Postgres, or BerkDB) backend