Add docs/DNS-mode-challenge.md

docs/DNS-mode-challenge.md

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+## DNS mode challenge configuration
+
+To use DNS mode challenge, define `LETSENCRYPT_DNS_MODE` and `LETSENCRYPT_DNS_MODE_SETTINGS` environment variables in the client container:
+
+	  environment:
+	  - LETSENCRYPT_DNS_MODE=<dns_provider_script_name>
+	  - LETSENCRYPT_DNS_MODE_SETTINGS=export <provider_setting>=<value> ...
+
+You find the `<dns_provider_script_name>` by browsing the `acme.sh` `dnsapi` folder [1]. the variable must hold the script name without the `.sh` extension.
+
+[1] https://github.com/acmesh-official/acme.sh/tree/master/dnsapi
+
+Examples:
+
+* To use Gandi LiveDNS:
+
+		- LETSENCRYPT_DNS_MODE=dns_gandi_livedns
+* To use DuckDNS:
+
+		- LETSENCRYPT_DNS_MODE=dns_duckdns
+
+To find about a provider's settings, read the comments provided at the begining of the related script.
+
+Example:
+
+* `dnsapi/dns_gandi_livedns.sh` has this comment:
+
+		# Requires GANDI API KEY set in GANDI_LIVEDNS_KEY set as environment variable
+   Then the settings would be:
+
+		- LETSENCRYPT_DNS_MODE_SETTINGS=export GANDI_LIVEDNS_KEY=<your_gandi_api_key>	
+* `dnsapi/dns_duckdns.sh` has this comment:
+
+		# export DuckDNS_Token="aaaaaaaa-bbbb-cccc-dddd-eeeeeeeeeeee"
+   Then the settings would be:
+
+		- LETSENCRYPT_DNS_MODE_SETTINGS=export DuckDNS_Token=<your_duckdns_token>