Contribution Guidelines out of date. No src folder

[Thomas Kagan]

The documentation at https://github.com/Addepar/ember-charts/blob/master/CONTRIBUTING.md#writing-code-in-ember-charts

References the src directory which contains the ember-charts source code but no src directory exists in the repo.

[Michael Ratanapintha]

Hello @kagant15! I apologize for the late response to this.

Please refer to pull request #170 which should fix this doc bug.

[Michael Ratanapintha]

Fixed with merge of #170; closing.