Add some notes about formatting

CONTRIBUTING.md

@@ -4,7 +4,15 @@
 
 Before a pull request can be merged, you need to to sign the [Dropbox Contributor License Agreement](https://opensource.dropbox.com/cla/).
 
-Please make sure to run the tests (`make check`) on your changes.
+Please make sure to run the tests (`make check` and `make check_format`) on your changes.
+
+##### Formatting
+
+Please make sure `make check_format` passes on your commits.  If it reports any issues, you can run `make format` to auto-apply
+the project formatting rules.  Note that this will format your files in-place with no built-in undo, so you may want to
+create a temporary commit if you have any unstaged changes.
+
+### Projects
 
 If you don't know where to start:
 - Check out the list of [starter projects](https://github.com/dropbox/pyston/wiki/Starter-projects)

src/Makefile

@@ -309,7 +309,8 @@ endef
 format:
 	find \( -name '*.cpp' -o -name '*.h' \) -print0 | xargs -0 $(LLVM_BIN)/clang-format -style=file -i
 check_format:
-	$(TOOLS_DIR)/check_format.sh $(LLVM_BIN)/clang-format
+	$(ECHO) checking formatting...
+	$(VERB) $(TOOLS_DIR)/check_format.sh $(LLVM_BIN)/clang-format
 
 .PHONY: test test_debug test_prof test_release
 test_debug: pyston_dbg ext

tools/check_format.sh

 #!/bin/bash
 set -eu
+failed=0
 for fn in $(find -name '*.cpp' -o -name '*.h'); do
-    $1 -style=file -output-replacements-xml $fn | grep -q "replacement offset" && { echo $fn "failed clang-format check"; exit 1; }
+    $1 -style=file -output-replacements-xml $fn | grep -q "replacement offset" && { echo $fn "failed clang-format check"; failed=1; }
 done
-echo "Format checks passed"
-exit 0
+
+if [ $failed -eq 0 ]; then
+    echo "Format checks passed"
+    exit 0
+else
+    exit 1
+fi