Revise explanation for closed-contribution

Contributing.md

@@ -1,14 +1,17 @@
 # Contributing
 
-I (Andrew Belt) generally do not accept external contributions to the Rack codebase itself.
+I (Andrew Belt) generally do not accept unpaid contributions to the Rack codebase itself.
 
-Before writing a single line of code, most features and bug fixes require
+Writing code is easy. Instead, the difficulty of changing Rack's source code involves
 - research to devise the best solution with a convincing argument
-- testing on relevant platforms, audio/MIDI devices, different plugins, etc.
+- acceptance of API/ABI change proposals
+- testing on relevant platforms, hardware devices, plugins, etc.
 - dedication to maintain the code in the future
 - generalizability to solve similar issues, and flexibility for solving future solutions without an entire rewrite
 - sometimes legal review
 
+Unpaid contributions typically omit many of the above tasks, making the code more expensive for me to accept than if I had written it myself.
+
 Instead, there are many other areas where contributions are much appreciated.
 - Dependencies of Rack. Especially [nanovg](https://github.com/memononen/nanovg)'s performance, [rtaudio](https://github.com/thestk/rtaudio)/[rtmidi](https://github.com/thestk/rtmidi)'s stability and compatibility, and maybe even touch support in [GLFW](https://github.com/glfw/glfw). You would be helping many more projects than just Rack.
 - Your own Rack [plugin](PluginDevelopmentTutorial.html)