Added some documentation in README

README.md

@@ -11,6 +11,12 @@ Written in Python, it needs at least python 3.2 for the ThreadPools.
 
 On Debian Wheezy the package is `python3`.
 
+Some checks are implemented using Nagios' checks,
+available under Debian in package `nagios-plugins-basic`.
+
+The DNS zone check calls [Bortzmeyer's](https://github.com/bortzmeyer/check_dns_soa)
+(introduced [here](http://www.bortzmeyer.org/go-dns-icinga.html)).
+
 
 Usage
 -----
@@ -25,13 +31,22 @@ Usage
 Configuration
 -------------
 
-Everything can be configured from config.py: base tick, hosts and checks.
+Everything can be configured from config.py: notified email(s), base tick, hosts and checks.
 
 The `base_tick` option defines the base granularity (in seconds) for polling.
 `Host`s are shorthands to avoid repeating addresses within checks.
 Checks are then defined as a list of `Check`-derived class instances that
 implement every single check (see lib/checks.py).
 
+Each check may be initialised with some options:
+
+  * `every`: run every `every` × `base_tick` seconds;
+  * `retry`: number of retries before considering a failure (so failure is after `every` × (`retry`+1) × `base_tick` seconds;
+  * `timeout`: subcommand timeout, to avoid stalling checks (defaults to 2 seconds);
+  * `target_name`: human-readable name of the target of the check (automatically set by the `name` option if using `Host` instances).
+
+In addition some checks have specific options, see lib/checks.py for examples.
+
 
 Current state output
 --------------------