Add LICENSE file to project

We need to add licensing information for some of the third party resources that we use. According to Google Material icons we should be including this apache license somewhere in our project.

https://www.apache.org/licenses/LICENSE-2.0.html

The license is linked to from this page:

https://material.io/resources/icons/?style=baseline

Here is an example of a LICENSE.md file in another project:

https://github.com/usgs/earthquake-event-ws/blob/master/LICENSE.md