Defer Linux support

https://github.com/golang/go/blob/master/src/crypto/x509/root_linux.go
https://github.com/golang/go/blob/master/src/crypto/x509/root_unix.go
https://www.brightbox.com/blog/2014/03/04/add-cacert-ubuntu-debian/
https://chromium.googlesource.com/chromium/src/+/lkcr/docs/linux_cert_management.md

README.md

@@ -34,7 +34,7 @@ On macOS, use Homebrew.
 brew install --HEAD FiloSottile/mkcert/mkcert
 ```
 
-On Linux, use [the pre-built binaries](https://github.com/FiloSottile/mkcert/releases), or build from source.
+On Linux (support coming soon!), use [the pre-built binaries](https://github.com/FiloSottile/mkcert/releases), or build from source.
 
 ```
 $ git clone https://github.com/FiloSottile/mkcert