Add ZSTD_defaultCLevel() function to public API

doc/zstd_manual.html

@@ -1441,6 +1441,11 @@ size_t ZSTD_freeCCtxParams(ZSTD_CCtx_params* params);
   which have troubles handling structures containing memory pointers.
  
 </p></pre><BR>
+  
+<pre><b>int ZSTD_defaultCLevel();
+</b><p> Returns the default compression level, specified by ZSTD_CLEVEL_DEFAULT
+
+</p></pre><BR>
 
 <a name="Chapter18"></a><h2>Advanced decompression functions</h2><pre></pre>