Add "Templating" section to the README

Author: md5

README.md

@@ -95,6 +95,25 @@ If no `<dest>` file is specified, the output is sent to stdout.  Mainly useful f
 
 ===
 
+### Templating
+
+The templates used by nginx-proxy are written the Go [text/template](http://golang.org/pkg/text/template/) language. In addition to the [built-in functions](http://golang.org/pkg/text/template/#hdr-Functions) supplied by Go, nginx-proxy provides a number of additional functions to make it simpler (or possible) to generate your desired output.
+
+#### Functions
+
+* *`contains $map $key`*: Returns `true` if `$map` contains `$key`. Takes maps from `string` to `string`.
+* *`exists $path`*: Returns `true` if `$path` refers to an existing file or directory. Takes a string.
+* *`groupBy $containers $fieldPath`*: Groups an array of `RuntimeContainer` instances based on the values of a field path expression `$fieldPath`. A field path expression is a dot-delimited list of map keys or struct member names specifying the path from container to a nested value, which must be a string. Returns a map from the value of the field path expression to an array of containers having that value. Containers that do not have a value for the field path in question are omitted.
+* *`groupByMulti $containers $fieldPath $sep`*: Like `groupBy`, but the string value specified by `$fieldPath` is first split by `$sep` into a list of strings. A container whose `$fieldPath` value contains a list of strings will show up in the map output under each of those strings.
+* *`split $string $sep`*: Splits `$string` into a slice of substrings delimited by `$sep`. Alias for [`strings.Split`](http://golang.org/pkg/strings/#Split)
+* *`replace $string $old $new $count`*: Replaces up to `$count` occurences of `$old` with `$new` in `$string`. Alias for [`strings.Replace`](http://golang.org/pkg/strings/#Replace)
+* *`dict $key $value ...`*: Creates a map from a list of pairs. Each `$key` value must be a `string`, but the `$value` can be any type (or `nil`). Useful for passing more than one value as a pipeline context to subtemplates.
+* *`sha1 $string`*: Returns the hexadecimal representation of the SHA1 hash of `$string`.
+* *`json $value`*: Returns the JSON representation of `$value` as a `string`.
+* *`last $array`*: Returns the last value of an array.
+
+===
+
 ### Examples
 
 * [Automated Nginx Reverse Proxy for Docker](http://jasonwilder.com/blog/2014/03/25/automated-nginx-reverse-proxy-for-docker/)