Add a Patterns section to the README.

The documentation on how the DNS serial number have to be formated was
missing. Even if the pattern is very common, it is important to specify
it.

README.md

@@ -25,6 +25,20 @@ Usage
 By default, each time you save a `bindzone` file, the script will look for the DNS serial number and update it. You can also update it without saving the file by invoking the `:DNSSerialUpdate` function.
 
 
+Patterns
+--------
+
+In order to be detected, the DNS serial number must match the following pattern:
+
+* `YYYYMMDDXX ; serial`
+  - `YYYY` is the year (4 digits);
+  - `MM` is the month (2 digits);
+  - `DD` is the day (2 digits);
+  - `XX` is any non-negative number (1 or more digits);
+  - the word `serial` is not case-sensitive;
+  - there can be any number of blanks on each sides of the semicolon.
+
+
 Configuration
 -------------