[center.conan.io] add badge for projects in conan center

[gocarlos]

would be great if cci automatically creates a badge for projects which they can put in the readme.

this badge should be a link to the https://conan.io/center/

this is already possible for bintray but not for conan center

[claremacrae]

As a manual workaround, the following markdown code works nicely:

[![ConanCenter package](https://repology.org/badge/version-for-repo/conancenter/approvaltests.cpp.svg)](https://conan.io/center/approvaltests.cpp)

It currently generates:

[Croydon]

This become possible at some point

See the badges tab: https://conan.io/center/recipes/openssl

Issue can be closed