Update Doxygen Documentation

 * Update README with notification about dropping support
   for Ubuntu 16.04
 * Update cross-compile guide

Signed-off-by: Jan Eilers <jan.eilers@arm.com>
Change-Id: I10b89ba09bc5737012557eacdeaacc620ccdb8a9
diff --git a/21.02/_open_cl_timer_8hpp_source.xhtml b/21.02/_open_cl_timer_8hpp_source.xhtml
index 5123573..aa0a76b 100644
--- a/21.02/_open_cl_timer_8hpp_source.xhtml
+++ b/21.02/_open_cl_timer_8hpp_source.xhtml
@@ -115,7 +115,7 @@
 <div id="nav-path" class="navpath"><!-- id is needed for treeview function! -->
   <ul>
     <li class="navelem"><a class="el" href="dir_68267d1309a1af8e8297ef4c3efbcdba.xhtml">src</a></li><li class="navelem"><a class="el" href="dir_0f3cdec46afbc61a1ded8e1687c9c9a0.xhtml">backends</a></li><li class="navelem"><a class="el" href="dir_1ad86c6d39ab715a831555571b9e98a5.xhtml">cl</a></li><li class="navelem"><a class="el" href="_open_cl_timer_8hpp.xhtml">OpenClTimer.hpp</a></li>
-    <li class="footer">Generated on Thu Feb 25 2021 17:27:51 for ArmNN by
+    <li class="footer">Generated on Fri Mar 19 2021 15:26:02 for ArmNN by
     <a href="http://www.doxygen.org/index.html">
     <img class="footer" src="doxygen.png" alt="doxygen"/></a> 1.8.13 </li>
   </ul>