[Docs] Update repo documents (#417)

* Add missing contributions and license files to this repository
* Improve readme with license and notice links

CODE_OF_CONDUCT.md

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+303 See Other
+
+Location: https://www.elastic.co/community/codeofconduct

CONTRIBUTING.md

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+# Contributing
+
+The Elasticsearch Go client is open source and we love to receive contributions from our community.
+
+There are many ways to contribute, from writing tutorials or blog posts, improving the documentation, submitting bug reports and feature requests or writing code.
+
+## Sign the Contributor License Agreement
+
+We do ask that you sign the [Contiributor License Agreement](https://www.elastic.co/contributor-agreement)
+before we can accept pull requests from you.
+
+## Development
+
+### Prerequisites
+
+You need at least Go 1.13 to build the project.
+
+Docker is used for some tests to spawn an Elasticsearch server.
+
+### Project structure
+
+The code for each API lives in individual files in the `esapi` subfolder, this code is entirely generated by the subproject located in the `internal/build` folder.
+
+
+
+### Before you open a PR
+
+Please run the tests locally with `make test` and make sure everything is in order.
+
+If your changes address the generator, you can run the integration with the following command:
+
+`TEST_SUITE=free STACK_VERSION=8.0.0-SNAPSHOT WORKSPACE=/tmp/workspace/$STACK_VERSION .ci/run-tests`
+
+TEST_SUITE can either be `free` or `platinum`, STACK_VERSION reflects the targeted Elasticsearch version of the client.

NOTICE

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+Elasticsearch Go Client
+Copyright 2021 Elasticsearch B.V.

README.md

@@ -375,4 +375,4 @@ The **[`_examples`](./_examples)** folder contains a number of recipes and compr
 
 ## License
 
-(c) 2019 Elasticsearch. Licensed under the Apache License, Version 2.0.
+This software is licensed under the [Apache 2 license](./LICENSE). See [NOTICE](./NOTICE).