Added note about ECJ split for JDT migration guide

See eclipse-jdt/eclipse.jdt.core#181

bundles/org.eclipse.jdt.doc.isv/porting/4.27/incompatibilities.html

@@ -15,5 +15,51 @@ <h1>Incompatibilities between Eclipse JDT 4.26 and 4.27</h1>
   plug-ins. Plug-ins that ran on 4.26 should run on 4.27 without any problems.
 </p>
 
+<!-- ############################################## -->
+<h3><a name="ecj_split">Note about ECJ separation from JDT Core</a></h3>
+<p>
+  ECJ (Eclipse compiler for Java) code is moved from <code>org.eclipse.jdt.core</code>
+  to dedicated <code>org.eclipse.jdt.core.compiler.batch</code> bundle and will be
+  deployed as a separated bundle.
+  The <code>org.eclipse.jdt.core.compiler.batch</code> is now included in SDK
+  as a regular Eclipse bundle and can be compiled / deployed / used separately
+  from <code>org.eclipse.jdt.core</code> bundle.
+  All of ECJ packages are re-exported by <code>org.eclipse.jdt.core</code>, therefore
+  from OSGI point of view, all 3rd party code that used some compiler related API 
+  from <code>org.eclipse.jdt.core</code> doesn't require any change.
+  The <code>org.eclipse.jdt.core.compiler.batch</code> bundle itself doesn't have any dependencies
+  and so can be used in Eclipse products that do not use workspace concepts.
+</p>
+<p>
+  However, no change is without side effects.
+</p>
+<p>
+  <b>Known problems with the split of the ECJ from core bundle</b>
+</p>
+<ol>
+  <li>
+  As part of the <code>org.eclipse.jdt.core.compiler.batch</code> code separation from 
+  <code>org.eclipse.jdt.core</code>, the two fragments of <code>org.eclipse.jdt.core</code> - 
+  <code>org.eclipse.jdt.compiler.apt</code> and <code>org.eclipse.jdt.compiler.tool</code>
+  were merged into <code>org.eclipse.jdt.core.compiler.batch</code>.
+  So if some target definition, launch configuration or build file referenced the two fragments, these
+  references can and should be removed now.
+  </li>
+  <li>
+  Another issue might affect standalone (non OSGI based) applications that
+  were using <code>org.eclipse.jdt.core</code> as a "simple" Java library 
+  (which jdt.core never was).
+  So for example code that had <code>org.eclipse.jdt.core_XYZ.jar</code> on 
+  classpath and tried to call this outside Eclipse:
+  <pre>ASTParser parser = ASTParser.newParser(AST.getJLSLatest());</pre>
+  would fail now with <pre>NoClassDefFoundError: org.eclipse.jdt.internal.compiler.env.ICompilationUnit</pre> 
+  because <code>org.eclipse.jdt.core.dom.ASTParser</code> uses internally ECJ 
+  APIs and they are now ... surprise ... moved to <code>org.eclipse.jdt.core.compiler.batch</code>
+  jar. To fix this error, <code>org.eclipse.jdt.core.compiler.batch</code> library
+  should be added to the application classpath.
+  </li>
+</ol>
+<!-- ############################################## -->
+
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