Documentation for semantic annotation extension to Scala IDE

Documentation for semantic annotation extension to Scala IDE (task #1001989, https://github.com/scala-ide/scala-ide/pull/644)

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It would be useful to specified the required dependencies. I was able to recreate the following list, but there is no need for everybody to do it themselves, specially because the compiler error messages are a bit obscure.

```
 org.scala-ide.sdt.core
 org.scala-lang.scala-library
 org.eclipse.jdt.ui
 org.eclipse.jface.text
 org.eclipse.jdt.core
 org.scala-ide.sdt.aspects
 org.scala-lang.scala-compiler
 org.scala-lang.scala-reflect;bundle-version="2.11.2"
```