documentation: Tell not to use make -jN

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

docs/buildroot.html

@@ -182,6 +182,12 @@
  $ make
 </pre>
 
+    <p>You <b>should never</b> use <code>make -jN</code> with
+    Buildroot: it does not support <i>top-level parallel
+    make</i>. Instead, use the <code>BR2_JLEVEL</code> option to tell
+    Buildroot to run each package compilation with <pre>make
+    -jN</pre>.</p>
+
     <p>This command will generally perform the following steps:</p>
     <ul>
       <li>Download source files (as required)</li>