Add an article about recent libraries deprecation.

We choose to drop unused, unpopular and not relevant part of Hoa and it's important to write about that.
Related to : hoaproject/Central#90

Source/Posts/2018/06-deprecate-unused-libraries.xyl

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+<?xml version="1.0" encoding="utf-8"?>
+<?xyl-overlay href="hoa://Application/Overlays/Article.xyl"?>
+<?xyl-meta name="title" value="Now, it's time to deprecate unused libraries."?>
+<?xyl-meta name="date" value="2018-06-15T19:00:18+01:00"?>
+
+<overlay xmlns="http://hoa-project.net/xyl/xylophone">
+  <article id="main">
+    <p>Hoa is a set of 61 PHP libraries, during our <a href="http://discourse.hoa-project.net/t/hoa-virtual-meeting/22/34">latest
+    Virtual Meeting</a> we choose to deprecate the libraries. This is important
+    for  us to ensure that Hoa stays relevant and up to date regarding real
+    world needs.<br/>
+    We choose to remove the ones that are:</p>
+    <ol>
+      <li>Un-used or unpopular,</li>
+      <li>Not relevant anymore because they refer to dead technologies.</li>
+    </ol>
+    <p>The main goal is to reduce the maintenance effort to maintain the Hoa
+    ecosystem.</p>
+    <p>To be sure about the list, we checked some packagist and github stats.
+    We also choose to keep the lib used internally at the moment.</p>
+    <p>Some of the libraries weren't registered on packagist which means that they
+    are not used in today PHP's world.</p>
+    <p>The deprecation process is simple:</p>
+    <ol>
+      <li>Abandon the project on Packagist,</li>
+      <li>Update the README.md file to inform users,</li>
+      <li>Archive the repository on Github.</li>
+    </ol>
+    <p>Here is the whole list of deprecated libraries:</p>
+    <ul>
+      <li><a href="https://github.com/hoaproject/cache">hoa/cache</a></li>
+      <li><a href="https://github.com/hoaproject/log">hoa/log</a></li>
+      <li><a href="https://github.com/hoaproject/memory">hoa/memory</a></li>
+      <li><a href="https://github.com/hoaproject/model">hoa/model</a></li>
+      <li><a href="https://github.com/hoaproject/notification">hoa/notification</a></li>
+      <li><a href="https://github.com/hoaproject/prototype">hoa/prototype</a></li>
+      <li><a href="https://github.com/hoaproject/serialize">hoa/serialize</a></li>
+      <li><a href="https://github.com/hoaproject/translate">hoa/translate</a></li>
+      <li><a href="https://github.com/hoaproject/tree">hoa/tree</a></li>
+      <li><a href="https://github.com/hoaproject/xmlrpc">hoa/xmlrpc</a></li>
+    </ul>
+    <p>This is important for us to ensure that Hoa stay relevant and up to date
+    regarding real world needs.</p>
+  </article>
+</overlay>