readme: explain what Buildroot provides in more detail

Author: cirosantilli

README.adoc

@@ -291,8 +291,8 @@ link:https://en.wikipedia.org/wiki/Buildroot[Buildroot] is a set of `make` scrip
 
 * GCC
 * Linux kernel
-* glibc
-* link:https://en.wikipedia.org/wiki/BusyBox[BusyBox]
+* C standard library: Buildroot supports several implementations, we use link:https://en.wikipedia.org/wiki/GNU_C_Library[glibc] by default
+* link:https://en.wikipedia.org/wiki/BusyBox[BusyBox]: provides the shell and basic command line utilities
 
 It therefore produces a pristine, blob-less, debuggable setup, where all moving parts are configured to work perfectly together.