docs: add nonECS image removal

[adnxn]

Summary

README and CHANGELOG updates for #1752

Implementation details

Testing

• Builds on Linux (make release)
• Builds on Windows (go build -out amazon-ecs-agent.exe ./agent)
• Unit tests on Linux (make test) pass
• Unit tests on Windows (go test -timeout=25s ./agent/...) pass
• Integration tests on Linux (make run-integ-tests) pass
• Integration tests on Windows (.\scripts\run-integ-tests.ps1) pass
• Functional tests on Linux (make run-functional-tests) pass
• Functional tests on Windows (.\scripts\run-functional-tests.ps1) pass

New tests cover the changes:

Description for the changelog

Licensing

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[adnxn]

for the following tests:

ecs/linux/unit_test
ecs/linux/unit_test_in_docker

the two unit test target failures are due to panic: test timed out after 25s which are being fixed separately. this is an intermittent failure that shows up in octokitten but not travis. we're in the process of removing the unit test runs from octokitten altogether and executing unit tests through travis only.

ecs/windows/functional_test
the functional test failure is same issue being tracked here #1763.

merging this PR to dev since it contains only documentation changes.