Add the NodeSchoolDC hexdex

Add the Washington DC hexdex that @diptajbasu created.

Author: robbiethegeek

hexdex.html

@@ -125,6 +125,9 @@ <h1 data-i18n="hexdex-h1">Hexdex</h1>
           <a class="imglink" href="https://github.com/nodeschool/campinas" title="Campinas">
             <img class="hex" src="/images/nodeschool-campinas.svg" alt="Campinas Hexagon">
           </a>
+          <a class="imglink" href="https://github.com/nodeschool/washingtondc" title="Washington DC">
+            <img class="hex" src="/images/nodeschool-washingtondc.svg" alt="Washington DC Hexagon">
+          </a>
           <img class="hex" src="/images/nodeschool-blank.png" alt="blank hexagon" aria-hidden="true">
       </div>
     </div>