Start changelog for 2.6

OpenKeychain/src/main/res/raw/help_changelog.html

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+<h2>2.6</h2>
+<ul>
+<li>key certifications (thanks to Vincent Breitmoser)</li>
+<li>new design for signature verification</li>
+<li>custom key length (thanks to Greg Witczak)</li>
+<li>fix share-functionality from other apps</li>
+</ul>
+
 <h2>2.5</h2>
 <ul>
 <li>fix decryption of symmetric pgp messages/files</li>