These contracts include the interfaces to extend the Spryker SDK. For more information on extending the Spryker SDK by implementing the interfaces defined in these contracts please refer to https://github.com/spryker-sdk/sdk/blob/master/docs/development.md#via-php-implementation
Usually, installation is not necessary on project level as the dependencies using this will have it as a dependency in their "require" section. But you can install it using Composer as follows:
composer require spryker-sdk/sdk-contracts