Ignore from Git possibly created symlink to 'dev/dags' for running 'a…

…irflow dags test' command (astronomer#1211)

For local development and testing, we create a symlink in the project
root for the `dev/dags` directory
for the `airflow dags test` command to discover dags when the
`AIRFLOW_HOME` is set to project dir.
This PR adds to `.gitignore` to ignore the symlink so that it does not
show as unstaged change
while working with `git`

.gitignore

@@ -160,3 +160,6 @@ webserver_config.py
 
 # VI
 *.sw[a-z]
+
+# Ignore possibly created symlink to `dev/dags` for running `airflow dags test` command.
+dags