Possible improvements to install instructions on the release page

Hi, the release page shows install instructions using a RUN command which requires curl, shasum etc… Another option would be to use something like this:

```
ADD --checksum=sha256:5adff01c5a797663948e656d2b61d10932369ee437eb5cb54fa872b2960f222b --chmod=755 \
https://github.com/aptible/supercronic/releases/download/v0.2.46/supercronic-linux-amd64 /usr/local/bin/supercronic
```

This way it should work in every container without installing software. I would have opened a PR, but couldn't find out from a quick "grep" via the online repo search how those release messages are generated. Feel free to just close the issue if you do not like the `ADD` approach.

Thank you for your work on supercronic either way 👍

Start by tracing how the release-page install instructions are generated, since the issue says an online repository search did not reveal the source. Compare the current RUN-based instructions with the proposed Docker ADD command and its checksum. Done means the release page uses an agreed installation approach that works without installing extra tools.

Written by the indexing model from the issue text.