refactor: Add Design.md to explain refactor

DESIGN.md

@@ -0,0 +1,99 @@
+# Introduction
+This library aims to simplify error handling in Go through the following key features
+* One unique function with argument for standard error and go-errors
+* Variadic options following the "Functional Options" pattern
+* An error code, obtained by concatenating identifiers defined at each calls of subfunctions, that allow tracing the root cause
+* Comply with error interface (Error() string)
+
+# One unique function
+The signature of this unique function would be:
+```go
+func Wrap(error, errors.Opts...) error
+```
+Where error can be of type 
+1. standard errors
+2. go-errors
+
+## standard errors
+The provided error will be considered as a root error and stored as cause of a new go-errors one
+
+## go-errors
+The provided error will be considered as a template for a new go-errors one
+
+# Variadic Options
+The variadic options would be the following one
+
+* `WithDetail`/`WithDetailf`: provide a message that details the error
+* `WithProperty`: provide a key/value pair for additional informations (filename, path, username, ... )
+* `WithIdentifier`: used to provide an identifer, it could be concatenated with other idenfier from previous calls from subfunctions (See example below for clarity)
+* `CausedBy`: used to trace a root error
+
+Signature could be as follow
+
+```go
+errors.WithDetail(string)
+errors.WithDetailf(string, ...any)
+errors.WithProperty(string, any)
+errors.WithIdentifier(int)
+errors.CausedBy(err error)
+```
+
+# Identifier
+
+Below an example of using `go-errors` with identifier.  
+Also, find the expected error message
+
+```go
+package main
+
+import (
+    "fmt"
+    "os"
+
+    errors "github.com/scality/go-errors"
+)
+
+var ErrForbidden = errors.New("forbidden")
+
+func main(){
+    err := call1()
+    fmt.Println(err)
+}
+
+func call1() error {
+	return errors.Wrap(
+        call2(),
+        errors.WithIdentifier(19),
+    )
+}
+
+func call2() error {
+	return errors.Wrap(
+        call3(),
+        errors.WithDetail("missing required role"),
+        errors.WithProperty("Role", "Reader"),
+        errors.WithProperty("User", "john.doe"),
+        errors.WithIdentifier(12),
+    )
+}
+
+func call3() error {
+    // ls -l test.txt:
+    // -rw------- 1 root root 5 Mar  5 08:18 test.txt
+    _, err := os.Open("test.txt")
+
+    // Something went wrong here
+	return errors.Wrap(
+        ErrForbidden,
+        errors.WithIdentifer(2),
+        errors.WithDetail("permission denied"),
+		errors.WithProperty("File", "test.txt"),
+        errors.CausedBy(err),
+    )
+}
+```
+
+In the following situation, the program would return
+```
+forbidden (19-12-2): permission denied: missing required role: Role='Reader', User='john.doe', File='test.txt', at=(func='main.call3', file='main.go', line='41'), caused by: open test.txt: permission denied
+```