doc: adding documentation for iff/elseiff/elsef

README.md

@@ -948,6 +948,31 @@ result := lo.If[int](false, 1).
 // 3
 ```
 
+Using callbacks:
+
+```go
+result := lo.IfF[int](true, func () int {
+        return 1
+    }).
+    ElseIfF(false, func () int {
+        return 2
+    }).
+    ElseF(func () int {
+        return 3
+    })
+// 1
+```
+
+Mixed:
+
+```go
+result := lo.IfF[int](true, func () int {
+        return 1
+    }).
+    Else(42)
+// 1
+```
+
 ### Switch / Case / Default
 
 ```go
@@ -986,6 +1011,17 @@ result := lo.Switch[int, string](1).
 // "1"
 ```
 
+Mixed:
+
+```go
+result := lo.Switch[int, string](1).
+    CaseF(1, func() string {
+        return "1"
+    }).
+    Default("42")
+// "42"
+```
+
 ### ToPtr
 
 Returns a pointer copy of value.