add developer contribution guidelines

CONTRIBUTING.md

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+# Contributing
+
+When contributing to this repository, please first discuss the change you wish to make via issue, email, or any other 
+method with the owners of this repository before making a change.
+
+# How to contribute
+
+## The development lifecycle
+
+- Pull the latest content from the develop branch of this central repository (not your fork).
+- Create a branch off the `main` branch. Name the branch appropriately, either briefly summarizing the bug 
+(ex., spatil/click-cli-hotfix) or feature or simply use the issue number in the name (ex., spatil/issue-414-fix).
+- After completing work and testing locally, push the code to the appropriate branch on your fork.
+- In Github, create a pull request from the bug/feature branch of your fork to the develop branch of the central repository.
+
+> A CCDH engineer must review and accept your pull request. A code review (which happens with both the 
+contributor and the reviewer present) is required for contributing.
+
+## Development environment setup
+
+1. Install package dependencies.
+2. Clone the sheet2linkml package repository.
+
+```shell
+git clone https://github.com/cancerDHC/sheet2linkml.git
+```
+
+3. Create and activate the virtual environment.
+4. Run the following commands to build sheet2linkml and install the package along with all of its dependencies:
+
+```shell
+cd sheet2linkml  # change directory to sheet2linkml
+git checkout main  # switch to main branch of sheet2linkml
+poetry build # build source and wheel archives
+pip install dist/sheet2linkml-x.y.z-py3-none-any.whl  # install wheel file
+```
+
+5. To test new changes made to any of the modules within sheet2linkml, do the following:
+
+```shell
+# make changes to any files or modules
+pip uninstall sheet2linkml  # uninstall package
+poetry build
+pip install dist/sheet2linkml-x.y.z-py3-none-any.whl  # install wheel file
+```
+
+## Release process
+
+Once the code has been merged into the `main` branch on this repo, there are two processes that need to be completed 
+to ensure a release is complete.
+
+- You should create a GitHub tag, with the appropriate version number.
+- You should push the package to PyPI.
+
+### Release to Test PyPI _(optional)_
+The purpose of this section is to verify that the package looks and works as intended, by viewing it on Test PyPI and 
+installing the test version in a separate virtual environment.
+
+```shell
+poetry build   # build the package
+poetry config repositories.testpypi https://test.pypi.org/legacy/   # add Test PyPI as an alternate package repository
+poetry publish -r testpypi   # publish the package to Test PyPI
+```
+
+Installing:
+
+```shell
+pip install --index-url https://test.pypi.org/simple/
+```
+### Release to PyPI _(mandatory)_
+
+If the package looks great on Test PyPI and works well, the next step is to publish the package to PyPI:
+
+```shell
+poetry publish  # publish the package to PyPI
+```
+
+> You'll need to register for a PyPI account before uploading packages to the package index. Similarly for Test 
+PyPI as well.
+
+