adding test details to CONTRIBUTING.md

CONTRIBUTING.md

@@ -4,6 +4,7 @@ We welcome improvements, extensions, bug reports and bug fixes to VAtools!
 
 ## Bug Reports
 To make bug reports most useful, please include:
+
 - a brief summary of the problem - what did you expect, and what actually happened?
 - precise steps to reproduce the problem, and minimal VCF and annotation files on which the steps can be run, if possible.
 
@@ -13,3 +14,56 @@ Direct pull requests are just fine - fork the repository, create an new branch,
 Before investing time and effort, it is often best to create an issue in 
 the github repository reporting the bug and/or proposing the change, and we'll be happy to discuss the 
 details and feasibility with you. 
+
+## Running Tests
+The test suite uses Python's built-in `unittest` framework and requires Python 3.10 or newer.
+
+### 1. Get the code
+Clone the repository (or your fork of it) and move into the top-level directory, the one containing `setup.py`:
+
+```
+git clone https://github.com/griffithlab/VAtools.git
+cd VAtools
+```
+
+### 2. Create an isolated Python environment
+This keeps VAtools' dependencies separate from the rest of your system. Use either conda:
+
+```
+conda create -n vatools python=3.12
+conda activate vatools
+```
+
+or a virtual environment:
+
+```
+python3 -m venv .venv
+source .venv/bin/activate
+```
+
+### 3. Install VAtools in editable mode
+With the environment active, and from the top-level `VAtools` directory:
+
+```
+pip install -e .
+```
+
+The `-e` ("editable") flag installs VAtools as a link to your checkout instead of a copy, so changes you make to the code take effect immediately without reinstalling. This step also installs all dependencies, including `testfixtures`, which the tests need.
+
+### 4. Run the tests
+Still in the top-level directory, run the full suite:
+
+```
+python -m unittest discover -v
+```
+
+To run the tests for a single tool, or a single test, pass its module or test name:
+
+```
+python -m unittest tests.test_vep_annotation_reporter
+python -m unittest tests.test_vep_annotation_reporter.VcfExpressionEncoderTests.test_all_vep_fields
+```
+
+Test inputs and expected outputs are in `tests/test_data/`. Most tests run a tool and compare its output to the expected results in that directory, so if you change a tool's output, you'll have to update the matching expected files too.
+
+The same tests run automatically on GitHub Actions (`.github/workflows/tests.yml`) for every push and pull request to `master`, on Python 3.10 through 3.14. Please make sure they pass locally before opening a PR.