Add readme notes about versioning strategy and known issues (bcl.async)

[sungam3r]

          > Please consider changing the dependency from Microsoft.Extensions.Diagnostics.HealthChecks (>= 7.0.9) to Microsoft.Extensions.Diagnostics.HealthChecks (>= 6.0.3), or having separate targets for net6.0 and net7.0.

Well... Of course it's possible but I think it goes against current targeting design when v X.* versions of packages are targeted at v X.* version of NET.

ping @unaizorrilla @adamsitnik

Originally posted by @sungam3r in https://github.com/Xabaril/AspNetCore.Diagnostics.HealthChecks/issues/1975#issuecomment-1837576023