markdown improved a bit

docs_topics/02-dependencies.md

@@ -8,26 +8,26 @@ The dependencies differ slightly based on the environment you use, and the requi
 
       luarocks install luasocket
 
-* [**copas**](https://github.com/keplerproject/copas) module for asynchoneous IO. Copas is an advanced co-routine
+* [**copas**](https://github.com/keplerproject/copas) _[optional dependency]_ module for asynchoneous IO. Copas is an advanced co-routine
   scheduler with far more features than the included `ioloop`. For anything more than a few devices, or for devices which
   require network IO beyond mqtt alone, Copas is the better alternative. Copas is also pure-Lua, but has parallel network
   IO (as opposed to sequential network IO in `ioloop`), and has features like; threads, timers, locks, semaphores, and
   non-blocking clients for http(s), (s)ftp, and smtp.
 
       luarocks install copas
 
-* [**luasec**](https://github.com/brunoos/luasec) module for SSL/TLS based connections. This is optional and may be
+* [**luasec**](https://github.com/brunoos/luasec) _[optional dependency]_ module for SSL/TLS based connections. This is optional and may be
   skipped if you don't need secure network connections (e.g. broker is located in your local network). It's not listed
   in package dependencies, please install it manually like this:
 
       luarocks install luasec
 
-* [**LuaBitOp**](http://bitop.luajit.org/) library to perform bitwise operations, which is required only on
+* [**LuaBitOp**](http://bitop.luajit.org/) _[optional dependency]_ library to perform bitwise operations, which is required only on
   Lua 5.1. It's not listed in package dependencies, please install it manually like this:
 
       luarocks install luabitop
 
-* [**LuaLogging**](https://github.com/lunarmodules/lualogging/) to enable logging by the MQTT client. This is optional
+* [**LuaLogging**](https://github.com/lunarmodules/lualogging/) _[optional dependency]_ to enable logging by the MQTT client. This is optional
   but highly recommended for long running clients. This is a great debugging aid when developing your clients. Also when
   using OpenResty as your runtime, you'll definitely want to use this, see
   [openresty.lua](https://xhaskx.github.io/luamqtt/examples/openresty.lua.html) for an example.