add luarocks badge

Author: zhaozg

README.md

@@ -5,6 +5,7 @@ lua-openssl toolkit - A free, MIT-licensed OpenSSL binding for Lua.
 [![Total alerts](https://img.shields.io/lgtm/alerts/g/zhaozg/lua-openssl.svg?logo=lgtm&logoWidth=18)](https://lgtm.com/projects/g/zhaozg/lua-openssl/alerts/)
 [![Language grade: C/C++](https://img.shields.io/lgtm/grade/cpp/g/zhaozg/lua-openssl.svg?logo=lgtm&logoWidth=18)](https://lgtm.com/projects/g/zhaozg/lua-openssl/context:cpp)
 [![Coverage Status](https://coveralls.io/repos/github/zhaozg/lua-openssl/badge.svg?branch=master)](https://coveralls.io/github/zhaozg/lua-openssl?branch=master)
+[![luarocks](https://img.shields.io/luarocks/v/zhaozg/openssl)](https://luarocks.org/modules/zhaozg/openssl)
 
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