Update the LICENSE file to contain the license used (Apache-2.0) and add a NOTICE file referencing it

[hottea773]

What do these changes do?

This PR partially reverts the change made in https://github.com/aio-libs/yarl/commit/94faa90c3c3e6d8ac519fa4b6bee154bc090e34b to remove the text of the Apache license used. This brings this repository in line with Apache guidance on this:

Include a copy of the Apache License, typically in a file called LICENSE, in your work, and consider also including a NOTICE file that references the License.

This PR copies best practice as shown in Apache libraries, such as https://github.com/apache/zookeeper/blob/master/LICENSE.txt & https://github.com/apache/flink/blob/master/LICENSE as well as other non-apache libraries which use this license such as https://github.com/Activiti/Activiti/blob/develop/LICENSE.txt & https://github.com/GoogleCloudPlatform/PerfKitBenchmarker/blob/master/LICENSE.

Are there changes in behavior for the user?

No

Related issue number

Checklist

N/A

[asvetlov]

please hold the pr for short period

[mjpieters]

please hold the pr for short period

What’s the status of this PR now, @asvetlov?

[codecov[bot]]

Codecov Report

Merging #666 (bffd6a7) into master (bffd6a7) will not change coverage. The diff coverage is n/a.

:exclamation: Current head bffd6a7 differs from pull request most recent head 51dc7c2. Consider uploading reports for the commit 51dc7c2 to get more accurate results

@@           Coverage Diff           @@
##           master     #666   +/-   ##
=======================================
  Coverage   99.73%   99.73%           
=======================================
  Files           4        4           
  Lines         768      768           
  Branches      218      218           
=======================================
  Hits          766      766           
  Misses          2        2           

Flag	Coverage Δ
unit	99.60% <0.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

:mega: We’re building smart automated test selection to slash your CI/CD build times. Learn more

[mjpieters]

Sorry this took so long to get merged!