doc: added writing migrations guide

README.md

@@ -55,5 +55,39 @@ func createUser(r *http.Request, w http.ResponseWriter) {
 
 ```
 
+### Writing migrations
+A migration is a struct that performs a migration on either a request or a response, but not both. Here's an example:
+
+```go
+  type createUserRequestSplitNameMigration struct{} 
+
+  func (c *createUserRequestSplitNameMigration) Migrate(body []byte, h http.Header) ([]byte, http.Header, error) {
+    var oUser oldUser 
+    err := json.Unmarshal(body, &oUser)
+    if err != nil {
+      return nil, nil, err 
+    }
+
+    var nUser user 
+    nUser.Email = oUser.Email 
+
+    splitName := strings.Split(oUser.FullName, " ")
+    nUser.FirstName = splitName[0]
+    nUser.LastName = splitName[1]
+
+    body, err = json.Marshal(&nUser)
+    if err != nil {
+      return nil, nil, err 
+    }
+
+    return body, h, nil 
+  }
+```
+
+Notice from the above that the migration struct name follows a particular structure. The structure adopted is `{handlerName}{MigrationType}`. The `handlerName` refers to the exact name of your handler. For example, if you have a handler named `LoginUser`, any migration on this handler should start with `LoginUser`. It'll also be what we use in `VersionRequest` and `VersionResponse`. The `MigrationType` can be `Request` or `Response`. We use this field to determine if the migration should run on the request or the response payload. 
+
+This library doesn't support multiple transformations per version as of the time of this writing. For example, no handler can have multiple changes for each version update. 
+
+
 ## License
 MIT License