build: fix 'make help'

Summary:
This fixes the usage of `make help` in the top-level and subdirectories.

Signed-off-by: Austin Seipp <austin@well-typed.com>

Test Plan: It worked now and didn't before.

Reviewers: hvr

Reviewed By: hvr

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D692

Makefile

@@ -31,7 +31,7 @@ default : all
 # For help, type 'make help'
 .PHONY: help
 help:
-	@cat MAKEHELP
+	@cat MAKEHELP.md
 
 ifneq "$(filter maintainer-clean distclean clean help,$(MAKECMDGOALS))" ""
 -include mk/config.mk

mk/sub-makefile.mk

@@ -59,4 +59,4 @@ help : sub-help
 sub-help :
 	@echo "You are in subdirectory \"$(dir)\"."
 	@echo "Useful targets in this directory:"
-	@cat $(TOP)/SUBMAKEHELP
+	@sed '1,/Using `make` in subdirectories/d' $(TOP)/MAKEHELP.md