readme: install emulators when building with docker

[crazy-max]

fixes #260

Adds instruction to install emulators to be able to build with Docker. We don't encounter this issue on Docker Desktop as emulators are already installed. On CI we already install emulators with qemu action:

docker-credential-helpers/.github/workflows/build.yml

Lines 137 to 139 in bdd92dd

	-
	name: Set up QEMU
	uses: docker/setup-qemu-action@v2

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 54.68%. Comparing base (f8e94d9) to head (2103f1b).
Report is 73 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #286      +/-   ##
==========================================
- Coverage   55.23%   54.68%   -0.56%     
==========================================
  Files           9        9              
  Lines         668      673       +5     
==========================================
- Hits          369      368       -1     
- Misses        256      262       +6     
  Partials       43       43              

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

[thaJeztah]

LGTM