Improvements to the README

[helenmasters]

As a user new to the Adopt OpenJDK area the README needs to have more information in it. It would benefit from a section along the lines of 'what is in this repository and why would I want to use it?'

In addition I've been trying to follow the steps outlined and there are some changes required there too. In the section below there isn't a test directory, the instruction cd /test/TestConfig doesn't work as TestConfig is at the top level. In addition it would be useful to put in an example for the format of the platform_on_which_to_test. Ideally there would be a list of all the available platforms which are valid for running tests on (this could be a link out to another github project containing the releases).

cd /test/TestConfig
  export JAVA_BIN=/location_of_JVM_under_test
  export SPEC=platform_on_which_to_test
  export JAVA_VERSION=[SE80|SE90] (SE90 default value)
  make -f run_configure.mk

[helenmasters]

Please provide example settings for JAVA_BIN too, reference #5

[helenmasters]

As Shelley mentioned here #7 the dependencies don't get pulled in by default so this instruction needs to be added to the README.

[smlambert]

Closing this for now, as Helen's points have been addressed. We will need to do another pass of documentation improvements, but can create fresh issue listing those enhancements.