Improve README for FalconJX compiler

The current README file is sparse and needs more details. Here is proposed README file content:

Apache FalconJx Prototype Cross Compiler
====================

Note: steps 1-4 and 7 below describe how to get and build Apache Flex SDK, develop branch. If you already have the the develop branch of SDK skip these steps.

To get and build FalconJX compiler follow these steps:

1. Create a working directory for Apache Flex ("repo" below).

2. cd repo

3. Get bleading-edge of SDK:
git clone https://git-wip-us.apache.org/repos/asf/flex-sdk.git sdk
cd sdk
git checkout develop
cd ..

4. Get TLF
git clone https://git-wip-us.apache.org/repos/asf/flex-tlf.git tlf

5. Get Falcon and switch to develop branch:
git clone https://git-wip-us.apache.org/repos/asf/flex-falcon.git falcon
cd falcon
git checkout develop
cd ..

6. Get ASJS
git clone https://git-wip-us.apache.org/repos/asf/flex-asjs.git asjs
cd asjs
git checkout develop
cd ..

7. Build SDK. Follow instructions in sdk/README, then:
cd sdk
ant main (Select all default answers when prompted, i.e. just hit ENTER until the build finishes).
cd ..

8. Build Falcon compiler. Follow instructions in falcon/compiler/README, then:

cd falcon/compiler
ant -Dsdk.branch=path-to-repo/sdk
cd ../..

(Note: the compiler/build.xml assumes Flex SDK is in ../flex-sdk dir, which is not what we have created in step 3 above, so you need to supply correct dir to Ant via sdk.branch parameter).

9. Build FalconJX.

cd falcon/compiler.jx
ant
cd ../..

10. Build ASJS and FlexJSUI.swc:
cd asjs
ant
cd frameworks/as
ant