Include a nextstrain-pathogen.yaml file at the top level of the pathogen

This is currently an empty file to indicate the top level pathogen repo.
The inclusion of this file allows the nextstrain build invocation to work
from any directory regardless of runtime.

This is necessary so that rules in "ingest" can access the the external
directory "nextclade_data".

nextstrain-pathogen.yaml

@@ -0,0 +1,5 @@
+# This is currently an empty file to indicate the top level pathogen repo.
+# The inclusion of this file allows the Nextstrain CLI to run the
+# `nextstrain build` from any directory regardless of runtime.
+#
+# See https://github.com/nextstrain/cli/releases/tag/8.2.0 for more details.