Add LICENSE which will help placate Gitlab

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>

diff --git a/LICENSE b/LICENSE
new file mode 120000 (symlink)
index 0000000..faf3022
--- /dev/null
+++ b/LICENSE
@@ -0,0 +1 @@
+AGPL-3
\ No newline at end of file