docs: add doc for package formatter

docs/formatter.md

@@ -0,0 +1,53 @@
+# Formatter
+formatter contains some functions for data formatting.
+
+<div STYLE="page-break-after: always;"></div>
+
+## Source:
+
+[https://github.com/duke-git/lancet/blob/main/formatter/formatter.go](https://github.com/duke-git/lancet/blob/main/formatter/formatter.go)
+
+<div STYLE="page-break-after: always;"></div>
+
+## Usage:
+```go
+import (
+    "github.com/duke-git/lancet/formatter"
+)
+```
+
+<div STYLE="page-break-after: always;"></div>
+
+## Index
+- [Comma](#Comma)
+
+<div STYLE="page-break-after: always;"></div>
+
+## Documentation
+
+
+
+### <span id="Comma">Comma</span>
+<p>Add comma to number by every 3 numbers from right. ahead by symbol char.
+Param should be number or numberic string.</p>
+
+<b>Signature:</b>
+
+```go
+func Comma(v interface{}, symbol string) string
+```
+<b>Example:</b>
+
+```go
+package main
+
+import (
+    "fmt"
+    "github.com/duke-git/lancet/formatter"
+)
+
+func main() {
+    fmt.Println(formatter.Comma("12345", ""))   // "12,345"
+    fmt.Println(formatter.Comma(12345.67, "¥")) // "¥12,345.67"
+}
+```