Add docs about withLatestFrom in the README

Author: jdisho

README.md

@@ -74,9 +74,11 @@ github "CombineCommunity/CombineExt"
 
 This section outlines some of the custom operators CombineExt provides.
 
-### withLatestFrom
+## withLatestFrom
 
-Merges two publishers into a single publisher by combining each value from `self` with the _latest_ value from the second publisher, if any.
+### withLatestFrom(_:)
+
+Merges **two** publishers into a single publisher by combining each value from `self` with the _latest_ value from the second publisher, if any.
 
 ```swift
 let taps = PassthroughSubject<Void, Never>()
@@ -100,6 +102,78 @@ withLatestFrom: Hello
 withLatestFrom: World!
 ```
 
+------
+
+### withLatestFrom(_: _:)
+
+Merges **three** publishers into a single publisher by combining each value from `self` with the *latest* value from the second and third publisher, if any.
+
+```swift
+let taps = PassthroughSubject<Void, Never>()
+let strings = CurrentValueSubject<String, Never>("Hello")
+let ints = CurrentValueSubject<Int, Never>(0)
+
+taps
+  .withLatestFrom(strings, ints)
+  .sink(receiveValue: { print("withLatestFrom: \($0.0),\($0.1)") })
+
+taps.send()
+taps.send()
+strings.send("World!")
+taps.send()
+ints.send(1)
+ints.send(2)
+taps.send()
+```
+
+#### Output
+
+```none 
+withLatestFrom: Hello,0
+withLatestFrom: Hello,0
+withLatestFrom: World!,0
+withLatestFrom: World!,2
+```
+
+------
+
+### withLatestFrom(_: _: _:)
+
+Merges **four** publishers into a single publisher by combining each value from `self` with the *latest* value from the second, third and fourth publisher, if any.
+
+```swift
+let taps = PassthroughSubject<Void, Never>()
+let strings = CurrentValueSubject<String, Never>("Hello")
+let ints = CurrentValueSubject<Int, Never>(0)
+let bools = CurrentValueSubject<Int, Never>(true)
+
+taps
+  .withLatestFrom(strings, ints)
+  .sink(receiveValue: { print("withLatestFrom: \($0.0),\($0.1),\($0.2)") })
+
+taps.send()
+taps.send()
+strings.send("World!")
+bools.send(true)
+taps.send()
+ints.send(1)
+bools.send(true)
+bools.send(false)
+ints.send(2)
+taps.send()
+```
+
+#### Output
+
+```none
+withLatestFrom: Hello,0,true
+withLatestFrom: Hello,0,true
+withLatestFrom: World!,0,true
+withLatestFrom: World!,2,false
+```
+
+
+
 ------
 
 ### flatMapLatest