Merge pull request #1804 from sbt/wip/document-inc-features

Add missing notes for enhance bytecode feature.

notes/0.13.8/enhance-bytecode-feature.markdown

@@ -0,0 +1,35 @@
+  [1714]: https://github.com/sbt/sbt/issues/1714
+
+
+### Fixes with compatibility implications
+
+### Improvements
+
+### Bytecode Enhancers
+
+sbt 0.13.8 adds an extension point whereby users can effectively manipulate java bytecode (.class files) *before* the
+incremental compiler attempts to cache the classfile hashes.  This allows libraries like ebean to function with sbt
+without corrupting the compiler cache and rerunning compile every few seconds.
+
+This splits the compile task into several subTasks:
+
+1. `previousCompile`:  This task returns the previously persisted `Analysis` object for this project.
+2. `compileIncremental`: This is the core logic of compiling Scala/Java files together.   This task actually does the
+    work of compiling a project incrementally, including ensuring a minimum number of source files are compiled.
+    After this method, all .class files that would be generated by scalac + javac will be available.
+3. `manipulateByteCode`: This is a stub task which takes the `compileIncremental` result and returns it.
+    Plugins which need to manipulate bytecode are expected to override this task with their own implementation, ensuring
+    to call the previous behavior.
+4. `compile`:  This task depends on `manipulateBytecode` and then persists the `Analysis` object containing all
+    incremental compiler information.
+
+Here's an example of how to hook the new `manipulateBytecode` key in your own plugin:
+
+    manipulateBytecode in Compile := {
+      val previous = (manipulateBytecode in Compile).value
+      doManipulateBytecode(previous)  // Note: This must return a new Compiler.CompileResult with our changes.
+    }
+
+See [#1714][1714] for the full details of the implementation.
+
+### Bug fixes