Add Scaladex badge + Add instructions for other build tools

[windymelt]

• Added badge for Scaladex, famous index site for Scala libraries.
• Added instructions for Mill and Scala CLI, in addition to sbt.
  • Especially, Scala CLI instruction helps users for evaluating the library.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (f2463a5) 93.04% compared to head (b534dca) 93.04%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #36   +/-   ##
=======================================
  Coverage   93.04%   93.04%           
=======================================
  Files          12       12           
  Lines         417      417           
  Branches       11       11           
=======================================
  Hits          388      388           
  Misses         29       29           

Flag	Coverage Δ
3	93.04% <ø> (ø)

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

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[tarao]

@windymelt LGTM