fix(repository): put back concepts in readme

packages/repository/README.md

@@ -153,6 +153,161 @@ export class RepoApplication extends BootMixin(
 }
 ```
 
+## Concepts
+
+### Repository
+
+`Repository` represents a specialized `Service` interface that provides
+strong-typed data access (for example, CRUD) operations of a domain model
+against the underlying database or service.
+
+`Repository` can be defined and implemented by application developers. LoopBack
+ships a few predefined `Repository` interfaces for typical CRUD and KV
+operations. These `Repository` implementations leverage `Model` definition and
+`Datasource` configuration to fulfill the logic for data access.
+
+```js
+interface Repository<T extends Model> {}
+
+interface CustomerRepository extends Repository<Customer> {
+  find(filter?: Filter, options?: Options): Promise<Customer[]>;
+  findByEmail(email: string): Promise<Customer>;
+  // ...
+}
+```
+
+See more examples at:
+- [Repository/CrudRepository/EntityRepository](src/repository.ts)
+- [KVRepository](src/kv-repository.ts)
+
+### Model
+
+A model describes business domain objects, for example, `Customer`, `Address`,
+and `Order`. It usually defines a list of properties with name, type, and other
+constraints.
+
+Models can be used for data exchange on the wire or between different systems.
+For example, a JSON object conforming to the `Customer` model definition can be
+passed in REST/HTTP payload to create a new customer or stored in a document
+database such as MongoDB. Model definitions can also be mapped to other forms,
+such as relational database schema, XML schema, JSON schema, OpenAPI schema, or
+gRPC message definition, and vice versa.
+
+There are two subtly different types of models for domain objects:
+
+- Value Object: A domain object that does not have an identity (ID). Its
+  equality is based on the structural value. For example, `Address` can be
+  modeled as `Value Object` as two US addresses are equal if they have the same
+  street number, street name, city, and zip code values. For example:
+  ```json
+  {
+    "name": "Address",
+    "properties": {
+      "streetNum": "string",
+      "streetName": "string",
+      "city": "string",
+      "zipCode": "string"
+    }
+  }
+  ```
+
+- Entity: A domain object that has an identity (ID). Its equality is based on
+  the identity. For example, `Customer` can be modeled as `Entity` as each
+  customer should have a unique customer id. Two instances of `Customer` with
+  the same customer id are equal since they refer to the same customer. For
+  example:
+  ```json
+  {
+    "name": "Customer",
+    "properties": {
+      "id": "string",
+      "lastName": "string",
+      "firstName": "string",
+      "email": "string",
+      "address": "Address"
+    }
+  }
+  ```
+
+### Datasource
+
+`Datasource` is a named configuration of a connector. The configuration
+properties vary by connectors. For example, a datasource for `MySQL` needs to
+set the `connector` property to `loopback-connector-mysql` with settings as
+follows:
+
+```json
+{
+  "host": "localhost",
+  "port": 3306,
+  "user": "my-user",
+  "password": "my-password",
+  "database": "demo"
+}
+```
+
+When a `Datasource` is instantiated, the configuration properties will be used
+to initialize the connector to connect to the backend system.
+
+### Connector
+
+`Connector` is a provider that implements data access or api calls with a
+specific backend system, such as a database, a REST service, a SOAP Web Service,
+or a gRPC micro-service. It abstracts such interactions as a list of operations
+in the form of Node.js methods.
+
+Typically, a connector translates LoopBack query
+and mutation requests into native api calls supported by the underlying Node.js
+driver for the given backend. For example, a connector for `MySQL` will map
+`create` method to SQL INSERT statement, which can be executed through MySQL
+driver for Node.js.
+
+### Mixin
+
+`Mixin` is a way of building up classes from reusable components by combining
+simpler partial classes, which can be modeled as `Mixin`.
+
+For example, the mixins belows add methods and properties to a base class to
+create a new one.
+
+```ts
+import {Class} from '@loopback/repository';
+
+// Mixin as a function
+function timestampMixin<T extends Class<{}>>(Base: T) {
+  return class extends Base {
+    created: Date = new Date();
+    modified: Date = new Date();
+  };
+}
+
+// The base class
+class Customer {
+  id: string;
+  lastName: string;
+  firstName: string;
+}
+
+// Mix in timestamp
+const CustomerWithTS = timestampMixin(Customer);
+```
+
+### Type
+
+To support property and parameter typing, LoopBack 4 introduces an extensible
+typing system to capture the metadata and perform corresponding checks and
+coercion. The following types are supported out of box.
+
+- StringType
+- BooleanType
+- NumberType
+- DateType
+- BufferType
+- AnyType
+- ArrayType
+- UnionType
+- ObjectType
+
 ## Related resources
 
 - https://martinfowler.com/eaaCatalog/repository.html
@@ -161,7 +316,7 @@ export class RepoApplication extends BootMixin(
 
 ## Contributions
 
-- [Guidelines](https://github.com/strongloop/loopback-next/blob/master/docs/DEVELOPING.md)
+- [Guidelines](https://github.com/strongloop/loopback-next/blob/master/docs/site/DEVELOPING.md)
 - [Join the team](https://github.com/strongloop/loopback-next/issues/110)
 
 ## Tests