Add an example with transducers (#3)

* Add an example with transducers

* Fix doctest

src/LazyGroupBy.jl

@@ -296,7 +296,7 @@ function _import_docs()
         try
             include_dependency(path)
             str = read(path, String)
-            str = replace(str, r"^```julia"m => "```jldoctest")
+            str = replace(str, r"^```julia"m => "```jldoctest $name")
             @eval @doc $str $name
         catch err
             @error(

src/docs/reduce.md

@@ -21,3 +21,33 @@ Transducers.GroupByViewDict{Bool,Mean{Float64,EqualWeight},…} with 2 entries:
   false => Mean: n=3 | value=1.33333
   true  => Mean: n=7 | value=4.71429
 ```
+
+An example for calculating the minimum, maximum, and number of each
+group in one go:
+
+```julia
+julia> using Transducers
+
+julia> table = ((k = gcd(v, 42), v = v) for v in 1:100);
+
+julia> collect(Iterators.take(table, 5))  # preview
+5-element Array{NamedTuple{(:k, :v),Tuple{Int64,Int64}},1}:
+ (k = 1, v = 1)
+ (k = 2, v = 2)
+ (k = 3, v = 3)
+ (k = 2, v = 4)
+ (k = 1, v = 5)
+
+julia> counter = reducingfunction(Map(_ -> 1), +);
+
+julia> reduce.(TeeRF(min, max, counter), Map(x -> x.v), grouped(x -> x.k, table))
+Transducers.GroupByViewDict{Int64,Tuple{Int64,Int64,Int64},…} with 8 entries:
+  7  => (7, 91, 5)
+  14 => (14, 98, 5)
+  42 => (42, 84, 2)
+  2  => (2, 100, 29)
+  3  => (3, 99, 15)
+  21 => (21, 63, 2)
+  6  => (6, 96, 14)
+  1  => (1, 97, 28)
+```