docs: add users of testcontainers-go

[mdelapenya]

What does this PR do?

It creates the section for companies using the project, starting with Elastic

[gianarb]

Great idea, I can't find who but I remember other repositories using ADOPT.md or something like that. I like the idea to split this list from the README (we can mention that in the readme)

[mdelapenya]

Great idea, I can't find who but I remember other repositories using ADOPT.md or something like that. I like the idea to split this list from the README (we can mention that in the readme)

I copied what the Java folks are doing in their repo: https://github.com/testcontainers/testcontainers-java/blob/master/docs/index.md 😄

[codecov]

Codecov Report

Merging #345 (496caa5) into master (5963181) will increase coverage by 0.20%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #345      +/-   ##
==========================================
+ Coverage   61.32%   61.53%   +0.20%     
==========================================
  Files          15       15              
  Lines         993      993              
==========================================
+ Hits          609      611       +2     
+ Misses        292      290       -2     
  Partials       92       92              

Impacted Files	Coverage Δ
docker.go	63.99% <0.00%> (+0.43%)	⬆️

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[gianarb]

Thanks