release notes and semversion

[ivanitskiy]

Is your feature request related to a problem? Please describe

In order to simplify usage and report potential issues for the njs-acme project we need to publish versions and releases periodically when have changes worth releasing (new features, bug-fix, incompatible changes, etc.).

Describe the solution you'd like

We should follow the semantic version (reminder: <breaking change>.<backward-compatible>.<minor-or-bugfix>) and make releases

Follow simple steps:

• For a set of changes, we use tags and create a GH release where a "compiled" version of njs-acme as single a JS file can be found.
• Document which NJS version is tested and compatible against NGINX NJS in the readme.
• Maintain a changelog file
• Optionally, create a docker image with a semantic version tag of njs-acme release and publish (docker hub or GH registry TBD later)
• Optionally, publish to NPM