edit Flannel license info so that GitHub recognizes it

GitHub uses a library called Licensee to identify a project's license
type. It shows this information in the status bar and via the API if it
can unambiguously identify the license.

This commit leaves the LICENSE file as-is and updates three other files
so that Licensee correctly recognizes the LICENSE file as the only
license-related file. The README's "License" section has been renamed
to "Licensing" and license-check.sh has been renamed to header-check.sh.
In the Makefile, the references to license-check.sh have been renamed
accordingly.

Collectively, these changes allow Licensee to successfully identify the
license type of Flannel as Apache 2.0.

Signed-off-by: Andrea Kao <eirinikos@gmail.com>

Makefile

@@ -1,4 +1,4 @@
-.PHONY: test e2e-test cover gofmt gofmt-fix license-check clean tar.gz docker-push release docker-push-all flannel-git
+.PHONY: test e2e-test cover gofmt gofmt-fix header-check clean tar.gz docker-push release docker-push-all flannel-git
 
 # Registry used for publishing images
 REGISTRY?=quay.io/coreos/flannel
@@ -67,7 +67,7 @@ ifeq ($(ARCH),amd64)
 endif
 
 ### TESTING
-test: license-check gofmt
+test: header-check gofmt
 	# Run the unit tests
 	# NET_ADMIN capacity is required to do some network operation
 	# SYS_ADMIN capacity is required to create network namespace
@@ -89,8 +89,8 @@ cover:
 	go test -coverprofile cover.out $(PACKAGES_EXPANDED)
 	go tool cover -html=cover.out
 
-license-check:
-	./license-check.sh
+header-check:
+	./header-check.sh
 
 # Throw an error if gofmt finds problems.
 # "read" will return a failure return code if there is no output. This is inverted wth the "!"

README.md

@@ -65,7 +65,7 @@ See [CONTRIBUTING][contributing] for details on submitting patches and the contr
 
 See [reporting bugs][reporting] for details about reporting any issues.
 
-## License
+## Licensing
 
 Flannel is under the Apache 2.0 license. See the [LICENSE][license] file for details.