added documentation for HOOK_CHAIN config parameter

docs/hook_chain.md

+# HOOK_CHAIN
+
+If you want to deploy (and clean) all challenges for a single certificate in one hook call you can use `HOOK_CHAIN=yes` in your config.
+
+Calls to your hook script change in a way that instead of having only X parameters on deploy_challenge and clean_challenge it will have Y*X parameters,
+where Y is the number of domains in your cert, and you'll have to iterate over a set of X parameters at a time in your hook script.
+
+See below for an example on how the calls change:
+
+### HOOK_CHAIN="no" (default behaviour)
+```
+# INFO: Using main config file /etc/letsencrypt.sh/config.sh
+Processing lukas.im with alternative names: www.lukas.im
+ + Checking domain name(s) of existing cert... unchanged.
+ + Checking expire date of existing cert...
+ + Valid till Jul  7 20:54:00 2016 GMT (Longer than 30 days). Ignoring because renew was forced!
+ + Signing domains...
+ + Generating private key...
+ + Generating signing request...
+ + Requesting challenge for lukas.im...
+ + Requesting challenge for www.lukas.im...
+HOOK: deploy_challenge lukas.im blablabla blablabla.supersecure
+ + Responding to challenge for lukas.im...
+HOOK: clean_challenge lukas.im blablabla blablabla.supersecure
+ + Challenge is valid!
+HOOK: deploy_challenge www.lukas.im blublublu blublublu.supersecure
+ + Responding to challenge for www.lukas.im...
+HOOK: clean_challenge www.lukas.im blublublu blublublu.supersecure
+ + Challenge is valid!
+ + Requesting certificate...
+ + Checking certificate...
+ + Done!
+ + Creating fullchain.pem...
+HOOK: deploy_cert lukas.im /etc/letsencrypt.sh/certs/lukas.im/privkey.pem /etc/letsencrypt.sh/certs/lukas.im/cert.pem /etc/letsencrypt.sh/certs/lukas.im/fullchain.pem /etc/letsencrypt.sh/certs/lukas.im/chain.pem 1460152442
+ + Done!
+```
+
+### HOOK_CHAIN="yes"
+```
+# INFO: Using main config file /etc/letsencrypt.sh/config.sh
+Processing lukas.im with alternative names: www.lukas.im
+ + Checking domain name(s) of existing cert... unchanged.
+ + Checking expire date of existing cert...
+ + Valid till Jul  7 20:52:00 2016 GMT (Longer than 30 days). Ignoring because renew was forced!
+ + Signing domains...
+ + Generating private key...
+ + Generating signing request...
+ + Requesting challenge for lukas.im...
+ + Requesting challenge for www.lukas.im...
+HOOK: deploy_challenge lukas.im blablabla blablabla.supersecure www.lukas.im blublublu blublublu.supersecure
+ + Responding to challenge for lukas.im...
+ + Challenge is valid!
+ + Responding to challenge for www.lukas.im...
+ + Challenge is valid!
+HOOK: clean_challenge lukas.im blablabla blablabla.supersecure www.lukas.im blublublu blublublu.supersecure
+ + Requesting certificate...
+ + Checking certificate...
+ + Done!
+ + Creating fullchain.pem...
+HOOK: deploy_cert lukas.im /etc/letsencrypt.sh/certs/lukas.im/privkey.pem /etc/letsencrypt.sh/certs/lukas.im/cert.pem /etc/letsencrypt.sh/certs/lukas.im/fullchain.pem /etc/letsencrypt.sh/certs/lukas.im/chain.pem 1460152408
+ + Done!
+```
+