Add license and rules/standards for contributors.

[bwhm]

As substrate are planning on transitioning from GPLv3.0 to the Apacha 2.0 license, I suggest we just go ahead with the latter. It being more permissive, we would probably have to anyway in order to continue pulling from upstream.

In addition to add the license file itself, I suggest adding the following header to all files:

// Copyright 2019 the Joystream Contributors.
// Copyright 2017-2019 Parity Technologies (UK) Ltd.

// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at

//    http://www.apache.org/licenses/LICENSE-2.0

// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

[mnaamani]

As we are transitioning to monorepo, we can open this issue there.