Document building on FreeBSD via vagrant

doc/BUILDING.md

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+To test building on FreeBSD:
+
+  vagrant init freebsd/FreeBSD-10.3-RELEASE
+  vagrant up --provider virtualbox
+
+This may show an error related to MAC addresses, but just try again:
+
+  vagrant up
+  vagrant ssh
+  sudo pkg install git gmake
+  git clone https://github.com/raboof/nethogs
+  cd nethogs
+  gmake
+  sudo ./src/nethogs