Add a README with installation instructions

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README.md

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+# Gerrit release tools
+
+This repository contains tools to support Gerrit release management. The
+tools can be used standalone, independent of the gerrit branch checked out.
+
+## Prerequisites
+
+It is assumed that [pip](https://pypi.org/project/pip/) and
+[pipenv](https://docs.pipenv.org/) are installed.
+
+## Installation
+
+Installation is as simple as cloning the repository:
+
+```
+  git clone https://gerrit.googlesource.com/gerrit-release-tools
+```
+
+and then installing the dependencies with `pipenv`:
+
+```
+  cd gerrit-release-tools
+  pipenv install
+```
+
+## Usage
+
+To use the tools the environment must be activated:
+
+```
+  pipenv shell
+```
+
+Refer to the individual tools' help for further details.