Add checksums to release files

This is to address an unopened issue similar to:
gruntwork-io/terragrunt#610

The checksums of all binaries within `bin/` will be saved within
`SHA256SUMS`. The integrity of binaries can now be verified with this
command: `sha256sum --check --ignore-missing SHA256SUMS`.

In the future, Gruntwork should consider using a PGP key to
cryptographically sign this checksums file and to distribute the
signature along with the rest of the release files. Doing so will likely
require using private CI/CD infrastructure instead of CircleCI so that
Gruntwork may have full control over their private signing key.

.circleci/config.yml

@@ -49,6 +49,7 @@ jobs:
       - checkout
       - attach_workspace:
           at: /go/src/github.com/gruntwork-io/health-checker
+      - run: cd bin && sha256sum * > SHA256SUMS
       - run: upload-github-release-assets bin/*
 
 workflows: