Add Contributing Guidelines and Issue and Pull Request Template

Details

Before there was no contributing guidelines in Open food facts server repository. Seeing this I thought of adding it to the repository. It was highly appreciated by the owner of the repository and maintainers of the repository.

Components

The contributing guidelines are given below-

Join the development

Before you join development, please set up the project on your local machine, run it and go through >the application completely. Press on any button you can find and see where it leads to. Explore.You'll >be more familiar with what is where and might even get some cool ideas on how to improve various >aspects of the app.

If you would like to work on an issue, drop in a comment at the issue. If it is already assigned to >someone, but there is no sign of any work being done, please free to drop in a comment so that the >issue can be assigned to you if the previous assignee has dropped it entirely.

Contributing

When contributing to this repository, please first discuss the change you wish to make via issue, or >the official slack channel.

Pull Request Process

Ensure any install or build dependencies are removed before the end of the layer when doing a
build.

Check that there are no conflicts and your request passes Travis build. >Check the log of the pass test if it fails the build.

Give the description of the issue that you want to resolve in the pull request message. The format >of the commit message to be fixed - Fixes #[issue number] [Description of the issue] Example: >Fixes #529: Add toast warning in MainActivity.java

Wait for the maintainers to review your pull request and do the changes if requested.

Contributions Best Practices

Commits

Write clear meaningful git commit messages (Do read here)

Make sure your PR's description contains GitHub's special keyword references that automatically >close the related issue when the PR is merged. (For more info click here)

When you make very very minor changes to a PR of yours (like for example fixing a failing Travis >build or some small style corrections or minor changes requested by reviewers) make sure you >squash your commits afterward so that you don't have an absurd number of commits for a very small >fix. (Learn how to squash at here

When you're submitting a PR for a UI-related issue, it would be really awesome if you add a >screenshot of your change or a link to a deployment where it can be tested out along with your PR. It >makes it very easy for the reviewers and you'll also get reviews quicker.

Feature Requests and Bug Reports

When you file a feature request or when you are submitting a bug report to the [issue tracker]>(https://github.com/openfoodfacts/openfoodfacts-server/issues), make sure you add steps to >reproduce it. Especially if that bug is some weird/rare one.