Add note about stb to 3rd party credits in LICENSE and docs

docs/Credits.aml

@@ -95,6 +95,14 @@
 </externalLink>
   </para></listItem>
   
+  <listItem><para>
+  <externalLink>
+<linkText>stb</linkText>
+<linkAlternateText>Used to read and write various image formats.</linkAlternateText>
+<linkUri>https://github.com/nothings/stb</linkUri>
+</externalLink>
+  </para></listItem>
+  
   <listItem><para>
   <externalLink>
 <linkText>Sandcastle Help File Builder</linkText>