add env variables doc

ENV_VARIABLES.md

@@ -0,0 +1,46 @@
+# Environment Variables
+* Mandatory
+```
+NANGO_DB_HOST
+NANGO_DB_NAME
+NANGO_DB_USER
+NANGO_DB_PASSWORD
+NANGO_DB_PORT
+NANGO_DB_SSL
+NANGO_SERVER_URL
+NANGO_CALLBACK_URL
+TEMPORAL_ADDRESS
+TEMPORAL_NAMESPACE
+NODE_ENV
+NANGO_ENTERPRISE # set to true
+MAILGUN_API_KEY # provided by Nango
+PERSIST_SERVICE_URL
+```
+
+# Optional
+```
+NANGO_DATABASE_URL
+SERVER_PORT
+AWS_ACCESS_KEY_ID
+AWS_BUCKET_NAME
+AWS_REGION
+AWS_SECRET_ACCESS_KEY
+DD_API_KEY
+DD_APP_KEY
+DD_ENV
+DD_PROFILING_ENABLED
+DD_SITE
+DD_TRACE_AGENT_URL
+```
+
+## Recommended
+```
+NANGO_ENCRYPTION_KEY
+```
+
+# Required Files
+* Two files required by Temporal are expected to be mounted at `/etc/secrets/${TEMPORAL_NAMESPACE}`:
+```
+/etc/secrets/${TEMPORAL_NAMESPACE}.crt
+/etc/secrets/${TEMPORAL_NAMESPACE}.key
+```