Externals: add hidapi

Current master (d17db57b9d4354752e0af42f5f33007a42ef2906) because
Android support was added after the current release (0.8.0-rc1).

Externals/hidapi/HACKING.txt

@@ -0,0 +1,15 @@
+This file is mostly for the maintainer.
+
+1. Build hidapi.dll
+2. Build hidtest.exe in DEBUG and RELEASE
+3. Commit all
+
+4. Run the Following
+	export VERSION=0.1.0
+	export TAG_NAME=hidapi-$VERSION
+	git tag $TAG_NAME
+	git archive --format zip --prefix $TAG_NAME/ $TAG_NAME >../$TAG_NAME.zip
+5. Test the zip file.
+6. Run the following:
+	git push origin $TAG_NAME
+