Add support policy as stated by @bradfitz in golang/go#34958 (comment)

Author: tklauser

DragonFly-BSD.md

@@ -1,5 +1,13 @@
 # Go on DragonFly BSD
 
+From https://github.com/golang/go/issues/34958#issuecomment-543852995 :
+
+> Go's DragonFly support policy is that we support the latest stable
+> release primarily, but also try to keep DragonFly master passing, in
+> prep for it to become the latest stable release.
+>
+> But that does mean we need one more builder at the moment.
+
 | **Kernel version**      | **Architectures** | **Initial support version** | **Final support version** |
 |:------------------------|:------------------|:----------------------------|:--------------------------|
 | 4.6 or above            | amd64             | Go 1.8                      |                           |