Clarify runtime dependency in case of offline instrumentation (#445). (#445)

diff --git a/org.jacoco.doc/docroot/doc/faq.html b/org.jacoco.doc/docroot/doc/faq.html
index aa38a2d..0efbce5 100644
--- a/org.jacoco.doc/docroot/doc/faq.html
+++ b/org.jacoco.doc/docroot/doc/faq.html
@@ -141,6 +141,14 @@
   <code>-Xshareclasses:none</code>.
 </p>
 
+<h3>Why do I get an error "<code>ClassNotFoundException: org.jacoco.agent[...]Offline</code>"?</h3>
+<p>
+  If you use <a href="offline.html">offline instrumentation</a> the instrumented
+  classes get a direct dependency on the JaCoCo runtime. Therefore
+  <code>jacocoagent.jar</code> of the same JaCoCo version must be on the
+  classpath and accessible from by the instrumented classes. 
+</p>
+
 </div>
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