showcase: add stark logo

[Mallikki]

I'm submitting a...

[ ] Regression (a behavior that used to work and stopped working in a new release)
[ ] Bug report  
[x] Feature request
[ ] Documentation issue or request
[ ] Support request => Please do not submit support request here, instead see https://github.com/NationalBankBelgium/stark/blob/master/CONTRIBUTING.md#got-a-question-or-problem

Expected behavior

To integrate Stark Logo into Stark, we should :

• add it on github in showcase/src/assets-base and starter/src/assets-base along with the license
• use it in the README.md to make it look nicer (top and center)
• add it on the homepage of the showcase and starter
• recreate it as SVG

The logo should be under a creative commons license, to match that of Angular and respects its own license since we reuse it.