Added netlify _header settings for static folder

The intention here is to cache all static *.jpg, *.png files, normally gatsby generates these files into /static/*/*/*.png and /static/*/*/*.png structure, but let's start with something easier

_headers

@@ -0,0 +1,4 @@
+[[headers]]
+for = "/static/*"
+[headers.values]
+Cache-Control = "public, max-age=360000"