Changed README to README.md so it looks better on pub.dartlang.org.

pkg/analyzer-experimental/README

-This code is part of an experimental port of the Editor's analysis engine from
-Java to Dart. While we will continue to support the Java version of the analysis
-engine and the services built on it, we also intend to provide the same services
-to Dart-based applications. This is very early code and we expect it to change,
-possibly in significant ways. While we are eager to see other people make use
-of the analysis engine, we also want to be clear, in case you are interested in
-doing so, that the current API's should in no way be considered to be stable.
-
-In particular, this code was automatically translated from the Java
-implementation. The Java implementation that was translated is still under
-development and will continue to change over time. The translator that was used
-is still under development and the output produced by the translator will change
-over time. Therefore, the API presented by this code will change. In addition,
-any edits made to this code will be overwritten the next time we re-generate
-this code.
-
-If you are interested in using this code, despite the disclaimer above,
-fantastic! Please let the editor team know so that we can get a sense of the
-interest in it. Also, feel free to ask questions and make requests for
-additional functionality.