docs: create readme

README.md

@@ -0,0 +1,78 @@
+## Project goals
+
+This package provides functions and service to help you making authentication verifications. You can retrieve and decode bearer tokens, such as User access token or M2M token.
+
+## Usage
+
+### Retrieve token
+
+#### gRPC Context
+
+```go
+// context metadata => Authorization: Bearer ...
+
+token, err := visiauth.RetrieveTokenFromContext(ctx)
+if err != nil {
+  ...
+}
+```
+
+#### HTTP Request
+
+```go
+// request header => Authorization: Bearer ...
+
+token, err := visiauth.RetrieveTokenFromRequest(req)
+if err != nil {
+  ...
+}
+```
+
+#### PubSub message attribute
+
+```go
+// message attribute => Authorization: Bearer ...
+
+token, err := visiauth.RetrieveTokenFromPubSubMessageAttribute(req)
+if err != nil {
+  ...
+}
+```
+
+### Decode token
+
+```go
+service := visiauth.NewService(redis.NewJwkFetcher(), neo4j.NewUserRepository())
+
+identity, err = service.DecodeAccessToken(r.Context(), token)
+if err != nil {
+  ...
+}
+```
+
+## Identities
+
+Two types of identities are provided by service : User and Application.
+
+### User access token (User)
+
+If token was generated using Client credentials flow, identity will be a user.
+
+```go
+user, ok := identity.(*visiauth.User)
+if !ok {
+  ...
+}
+```
+
+### M2M Token (Application)
+
+If token was generated using Client ID and Client secret, identity will be an application.
+
+```go
+app, ok := identity.(*visiauth.App)
+if !ok {
+  ...
+}
+```
+