Update contribution guidelines

[maicki]

No description provided.

[CLAassistant]

All committers have signed the CLA.

[ghost]

Build failure:

[ghost]

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[ghost]

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[ghost]

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[garrettmoon]

Good to go once comments are addressed.

[garrettmoon]

Can you change to http://texturegroup.org/slack.html

[garrettmoon]

Can you change link to https://github.com/texturegroup/texture/issues?

[garrettmoon]

Update link to http://texturegroup.org/slack.html

[garrettmoon]

Here's the CLA link: https://cla-assistant.io/TextureGroup/Texture

[garrettmoon]

Apache 2. You can also say, see README.md for more details (I'll be updating it).

[garrettmoon]

Can you change the modified file to have this copyright header:

//  Copyright (c) 2014-present, Facebook, Inc.  All rights reserved.
//  This source code is licensed under the BSD-style license found in the
//  LICENSE file in the root directory of this source tree. An additional grant
//  of patent rights can be found in the PATENTS file in the same directory.
//  Modifications to this file made after 4/13/2017 are: Copyright (c) 2017-present,
//  Pinterest, Inc.  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.

[garrettmoon]

For the new files can you use this:

//  Copyright (c) 2017-present, Pinterest, Inc.  All rights reserved.
//  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.