docs: note musl based static builds, that work across linux distros

[YOU54F]

as per the new feature #62

it would be good to document this on the readme, so users are aware that

1. Alpine is now supported
2. The builds are static, and therefore the single binary works across musl or glibc based distros.

library	alpine support	version
pact_ffi	✅	- x86_64 .a from 0.2.4
		- aarch64 .a from 0.4.15
		- .so from 0.4.17
pact_mock_server_cli	✅	- 1.0.5
pact_verifier_cli	✅	- 1.1.1
pact-stub-server	✅	- 0.6.0
pact-plugin-cli	✅	- 0.1.2
pact-protobuf-plugin	✅	- 0.3.15
pact-csv-plugin	✅	- 0.0.6