Add aks-engine redirection notice to front page README

[jackfrancis]

What this PR does / why we need it: This commit will mark the official EOL of this codebase for future features and bug fixes!

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #

Special notes for your reviewer:

If applicable:

• [ ] documentation
• [ ] unit tests
• [ ] tested backward compatibility (ie. deploy with previous version, upgrade with this branch)

Release note:

Add aks-engine redirection notice to front page README

[mboersma]

/lgtm

[codecov[bot]]

Codecov Report

Merging #4335 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4335   +/-   ##
=======================================
  Coverage   55.45%   55.45%           
=======================================
  Files         109      109           
  Lines       16054    16054           
=======================================
  Hits         8903     8903           
  Misses       6368     6368           
  Partials      783      783

[codecov[bot]]

Codecov Report

Merging #4335 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4335   +/-   ##
=======================================
  Coverage   55.45%   55.45%           
=======================================
  Files         109      109           
  Lines       16054    16054           
=======================================
  Hits         8903     8903           
  Misses       6368     6368           
  Partials      783      783

[tariq1890]

/lgtm