boost: add link giving the list of architectures supported by context

To help understanding the list of architectures supported by
Boost.Context, add a link to the appropriate documentation page.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/boost/Config.in

@@ -65,6 +65,9 @@ config BR2_PACKAGE_BOOST_CHRONO
 config BR2_PACKAGE_BOOST_CONTAINER
 	bool "boost-container"
 
+# see
+# http://www.boost.org/doc/libs/1_59_0/libs/context/doc/html/context/architectures.html
+# for the list of supported architectures.
 config BR2_PACKAGE_BOOST_CONTEXT
 	bool "boost-context"
 	depends on (BR2_arm || BR2_armeb || BR2_i386 || BR2_mips || BR2_mipsel \