Adds a readme for the build tool script

BUILD-TOOL-README.md

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+# RV Shotgun plugin build tool
+[RV](https://www.shotgunsoftware.com/rv) is an image, audio and video review software 
+owned by Shotgun.
+
+This is a script to assist with RV's package development in order to iterate and deploy easily.
+
+
+### How it works
+It searches inside the `plugin` folder and adds the contents to a zip _.rvpkg_ file.
+It can then install and add the `rvpkg` to the RV installation.
+It can start/restart RV to pick up the latest changes.
+
+This allows a developer to make changes on their development environment and use the 
+script to deploy and restart to see those reflected on RV.
+
+##### Requirements
+
+* RV-2021.0.0
+* A `PACKAGE` yaml file following the RV convention inside the 'plugin' folder
+
+### Installation
+
+1. Clone repo
+2. Copy or write your plugin code inside the `plugin` folder, include a `PACKAGE` yaml 
+file and at least one `mu` or `python` file.
+3. To create the `rvpkg` file:
+    ```bash
+    python3 build-tool.py
+   ```
+4. To install your plugin on RV:
+    ```bash
+    python3 build-tool.py --install
+   ```
+5. Build, install and restart all current RV sessions:
+    ```bash
+    python3 build-tool.py --install --restart
+   ```
+
+
+#### Tools used and compatible versions
+
+* Written in Python 3
+* Tested with RV-2021.0.0
+* Aims to be completely cross-platform, however it's still a WIP. Please check the
+script to see which features are not implemented yet.
+
+
+
+----------------------------------------------------------------------------
+_Copyright 2020 Benteveo Ltd._
+
+_Licensed under the GNU General Public License v3.0_