added CONTRIBUTING.md file

Author: komalsharmaaa

CONTRIBUTING.md

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+# Contributing to terraform-provider-for-vmware-nsxt-virtual-private-cloud
+
+The terraform-provider-for-vmware-nsxt-virtual-private-cloud project team welcomes contributions from the community. If you wish to contribute code and you have not signed our contributor license agreement (CLA), our bot will update the issue when you open a Pull Request. For any questions about the CLA process, please refer to our [FAQ](https://cla.vmware.com/faq).
+
+## Contribution Flow
+
+This is a rough outline of what a contributor's workflow looks like:
+
+- Create a topic branch from where you want to base your work
+- Make commits of logical units
+- Make sure your commit messages are in the proper format (see below)
+- Push your changes to a topic branch in your fork of the repository
+- Submit a pull request
+
+Example:
+
+``` shell
+git remote add upstream https://github.com/vmware/terraform-provider-for-vmware-nsxt-virtual-private-cloud.git
+git checkout -b my-new-feature master
+git commit -a
+git push origin my-new-feature
+```
+
+### Staying In Sync With Upstream
+
+When your branch gets out of sync with the vmware/master branch, use the following to update:
+
+``` shell
+git checkout my-new-feature
+git fetch -a
+git pull --rebase upstream master
+git push --force-with-lease origin my-new-feature
+```
+
+### Updating pull requests
+
+If your PR fails to pass CI or needs changes based on code review, you'll most likely want to squash these changes into
+existing commits.
+
+If your pull request contains a single commit or your changes are related to the most recent commit, you can simply
+amend the commit.
+
+``` shell
+git add .
+git commit --amend
+git push --force-with-lease origin my-new-feature
+```
+
+If you need to squash changes into an earlier commit, you can use:
+
+``` shell
+git add .
+git commit --fixup <commit>
+git rebase -i --autosquash master
+git push --force-with-lease origin my-new-feature
+```
+
+Be sure to add a comment to the PR indicating your new changes are ready to review, as GitHub does not generate a
+notification when you git push.
+
+### Formatting Commit Messages
+
+We follow the conventions on [How to Write a Git Commit Message](http://chris.beams.io/posts/git-commit/).
+
+Be sure to include any related GitHub issue references in the commit message.  See
+[GFM syntax](https://guides.github.com/features/mastering-markdown/#GitHub-flavored-markdown) for referencing issues
+and commits.
+
+## Reporting Bugs and Creating Issues
+
+When opening a new issue, try to roughly follow the commit message format conventions above.