Post coverage details on coveralls and link to them in readme

Author: theotherjimmy

.travis.yml

@@ -24,6 +24,8 @@ script:
   - coverage run tools/project.py -S
   - coverage run tools/build_travis.py
   - coverage html
+after_success:
+  - coveralls
 before_install:
   - sudo add-apt-repository -y ppa:team-gcc-arm-embedded/ppa
   - sudo add-apt-repository -y ppa:libreoffice/libreoffice-4-2
@@ -40,3 +42,4 @@ install:
   - pip install hypothesis
   - pip install mock
   - pip install coverage
+  - pip install coveralls

README.md

@@ -21,6 +21,8 @@ We run continuous integration on all of our branches and pull requests to verify
  - Master branch [![Master Branch CI Badge](https://travis-ci.org/ARMmbed/mbed-os.svg?branch=master)](https://travis-ci.org/ARMmbed/mbed-os)
  - Latest release [![Latest Tag CI Badge](https://travis-ci.org/ARMmbed/mbed-os.svg?branch=latest)](https://travis-ci.org/ARMmbed/mbed-os/branches)
 
+Tools coverage [![Coverage Status](https://coveralls.io/repos/github/ARMmbed/mbed-os/badge.svg?branch=master)](https://coveralls.io/github/ARMmbed/mbed-os?branch=master)
+
 ## Getting Started for Developers
 
 You need [mbed CLI](https://github.com/ARMmbed/mbed-cli) to build mbed OS. For more details, read the [mbed OS Handbook](https://docs.mbed.com/docs/mbed-os-handbook/en/latest/).