Added a README for nightly (#1977)

nightly/README.md

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+# Nightly Testing Suite
+
+This is an infrastructure of nearcore end-to-end tests which is running in an
+endless loop on our CI.
+
+## Setup
+
+### Dependencies
+
+This testing suite is implemented in Python, so you will need to have Python
+installed (Python 3.8 is confirmed to work fine).
+
+Besides basic Python installation, extra Python packages are required, so it is
+recommended to use python-virtualenv, or pyenv, or similar projects to install
+those requirements:
+
+```
+$ cd /path-to-nearcore/
+$ pip install -r ./nightly/requirements.txt -r ./pytest/requirements.txt
+```
+
+### Nearcore
+
+The testing suite expects `nearcore` to be built before you run the suite, so
+make sure that everything is built:
+
+```
+$ cd /path-to-nearcore/
+$ cargo build --workspace --tests
+```
+
+## Run
+
+### a single test
+
+NOTE: Don't forget to activate your Python environment before running this.
+
+```
+$ cd /path-to-nearcore/pytest/
+$ python tests/sanity/block_production.py
+```
+
+### the whole suite
+
+NOTE: Don't forget to activate your Python environment before running this.
+
+```
+$ cd /path-to-nearcore/nightly/
+$ python nightly.py run nightly.txt /tmp/nearcore-testruns/$(git rev-parse HEAD)_$(date +%y%m%d_%H%M%S)
+```