Unit tests: update README.md

[lorjala]

Description

Refactor documentation and remove fluff.

Pull request type

[ ] Fix
[x] Refactor
[ ] Target update
[ ] Functionality change
[ ] Breaking change

[cmonr]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 3131
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/8212/

Triggering tests

/morph test
/morph export-build
/morph mbed2-build

[mbed-ci]

Test : FAILURE

Build number : 2933
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/8212/2933

[mbed-ci]

Exporter Build : SUCCESS

Build number : 2739
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/8212/

[cmonr]

/morph test

[mbed-ci]

Test : SUCCESS

Build number : 2939
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/8212/2939