Add Allocs example to README

[brenhinkeller]

People often complain that Julia doesn't have enough tools for tracking down allocations. This package, together with Profile.Allocs, actually seems pretty great for that to me though -- but you might not be able to figure out how from a casual glance at the README without watching the JuliaCon talk.

This PR adds a simple allocation profiling example that mirrors the top-line cpu profiling example just above it in the README.

This PR also updates the readme CI badge to point to the current github actions CI instead of the old Travis.

[codecov-commenter]

Codecov Report

Merging #76 (cab09e8) into master (82bebb5) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #76   +/-   ##
=======================================
  Coverage   96.61%   96.61%           
=======================================
  Files           3        3           
  Lines         295      295           
=======================================
  Hits          285      285           
  Misses         10       10           

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