Add a Requirements section in Build System docs

Author: eregon

docs/build_system.md

@@ -2,6 +2,17 @@
 
 There are many ways to build YARP, which means the build system is a bit more complicated than usual.
 
+## Requirements
+
+* It must work to build YARP for all 6 uses-cases below.
+* It must be possible to build YARP without needing ruby/rake/etc.
+  Because once YARP is the single parser in TruffleRuby, JRuby or CRuby there won't be another Ruby parser around to parse such Ruby code.
+  Most/every Ruby implementations want to avoid depending on another Ruby during the build process as that is very brittle.
+* It is desirable to compile YARP with the same or very similar compiler flags for all use-cases (e.g. optimization level, warning flags, etc).
+  Otherwise, there is the risk YARP does not work correctly with those different compiler flags.
+
+The main solution for the second point seems a Makefile, otherwise many of the usages would have to duplicate the logic to build YARP.
+
 ## General Design
 
 1. Templates are generated by `templates/template.rb`