Add doc for feature vendored-openssl in README.md

Signed-off-by: Jiahao XU <Jiahao_XU@outlook.com>

README.md

@@ -155,6 +155,8 @@ cargo build --release [--no-default-features --features=s3|redis|gcs|memcached|a
 
 By default, `sccache` builds with support for all storage backends, but individual backends may be disabled by resetting the list of features and enabling all the other backends. Refer the [Cargo Documentation](http://doc.crates.io/manifest.html#the-features-section) for details on how to select features with Cargo.
 
+Feature `vendored-openssl` can be used to statically link with openssl if feature openssl is enabled.
+
 ### Building portable binaries
 
 When building with the `dist-server` feature, `sccache` will depend on OpenSSL, which can be an annoyance if you want to distribute portable binaries. It is possible to statically link against OpenSSL using the `openssl/vendored` feature.