add luarocks for skywalking-nginx-lua. (#9)

luarocks is package manager for lua
Co-authored-by: NJared Tan <jian.tan@daocloud.io>

skywalking-nginx-lua-1.0-0.rockspec

+package = "skywalking-nginx-lua"
+version = "1.0-0"
+source = {
+   url = "git://github.com/apache/skywalking-nginx-lua",
+   branch = "master",
+}
+
+description = {
+   summary = "The Nginx Lua agent for Apache SkyWalking",
+   homepage = "https://github.com/apache/skywalking-nginx-lua",
+   license = "Apache License 2.0"
+}
+
+dependencies = {
+   "lua-resty-http = 0.15"
+}
+
+build = {
+   type = "builtin",
+   modules = {
+    ["skywalking.register"] = "lib/skywalking/register.lua",
+    ["skywalking.segment_ref"] = "lib/skywalking/segment_ref.lua",
+    ["skywalking.segment"] = "lib/skywalking/segment.lua",
+    ["skywalking.span_layer"] = "lib/skywalking/span_layer.lua",
+    ["skywalking.span"] = "lib/skywalking/span.lua",
+    ["skywalking.tracing_context"] = "lib/skywalking/tracing_context.lua",
+    ["skywalking.util"] = "lib/skywalking/util.lua",
+   }
+}