Update documents

Author: shibatch

docs/index.xhtml

@@ -267,8 +267,11 @@
 <p class="noindent">
 SLEEF is distributed under <a class="underlined"
 href="http://www.boost.org/LICENSE_1_0.txt">Boost Software License
-Version 1.0</a>. Contributions to this project are accepted under the
-same license.
+Version 1.0</a>. Contributions to this project
+from individual developers and small organizations are accepted under
+the same license. For contributions from corporate users, please refer
+to link: <a class="underlined"
+href="https://github.com/shibatch/sleef/blob/master/CONTRIBUTING.md">CONTRIBUTING.md</a>.
 </p>
 
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