Specify the OpenSSL dependency in the readme

README.md

@@ -23,6 +23,8 @@ The Automatic Certificate Management Environment (ACME), is an internet standard
 
 In order to compile ADMEd, you will need the [Rust](https://www.rust-lang.org/) compiler and its package manager, Cargo. The minimal required Rust version is 1.31.0, although it is recommended to use the latest stable one.
 
+ACMEd depends on the OpenSSL. The minimal supported versions are those from the [openssl](https://docs.rs/openssl/) crate, currently OpenSSL 1.0.1 through 1.1.1 and LibreSSL 2.5 through 2.8.
+
 ```
 cargo build --release
 ```