docs: add README banner about Vyper audit competition

[miohtama]

Add a temporary banner at the top of the README to advertise the audit competition

What I did

Advertise the audit competition in README

How I did it

Bold sentence and links

How to verify it

Commit message

Let's audit it

Description for the changelog

Cute Animal Picture

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    | | | u:u | | |
    | | \     / | |
jgs | | _|   | _| |
    / \""`   `""/ \
   | __|       | __|
   `"""`       `"""`

[codecov-commenter]

Codecov Report

Merging #3599 (a43c23f) into master (3b310d5) will not change coverage.
The diff coverage is n/a.

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@@           Coverage Diff           @@
##           master    #3599   +/-   ##
=======================================
  Coverage   89.09%   89.09%           
=======================================
  Files          85       85           
  Lines       11392    11392           
  Branches     2591     2591           
=======================================
  Hits        10150    10150           
  Misses        820      820           
  Partials      422      422           

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[miohtama]

The falling tests are likely to be unrelated as this change is README only