Explain why this doesn't have a full libc.

Author: sunfishcode

README.md

@@ -29,3 +29,20 @@ To use the sysroot, use the `--sysroot=` option:
 ```
 
 to run the compiler using the newly built sysroot.
+
+## Why doesn't this contain a full libc implementation?
+
+In the short term, one of the main goals is just to provide a reference
+point for people already maintaining their own libc codebases, to help
+reduce interface incompatibilities between the several different
+environments out there.
+
+In the long term, there may some day be some form of standardized
+syscall/import layer for wasm which would could support a full
+"reference libc", at which point this repository might make sense as
+a place to host such a thing.
+
+In between, if there are specific pieces of libc functionality which
+people would find useful to have here, and which don't depend on any
+syscalls, we could add them, using code from existing third-party
+codebases as appropriate.