Doc: Add readme for gateware builder.

README.md

+# BeagleV Fire Gateware Builder
+
+## Introduction
+The BeagleV Fire gateware builder is a Python script that builds both the PolarFire SoC HSS bootloader and Libero FPGA project into a single programming bitstream. It uses a list of repositories/branches specifying the configuration of the BeagleV Fire to build.
+
+
+## Prerequisites
+### Python libraries
+The following Python libraries are used:
+- GitPython
+- PyYAML
+
+```
+pip3 install gitpython
+pip3 install pyyaml
+```
+
+### Microchip Tools
+The SoftConsole and Libero tools from Microchip are required by the bitstream builder.
+
+The following environment variables are required for the bitstream builder to use the Microchip tools:
+- SC_INSTALL_DIR
+- FPGENPROG
+- LIBERO_INSTALL_DIR
+- LM_LICENSE_FILE
+
+An example script for setting up the environment is available [here](https://git.beagleboard.org/beaglev-fire/Microchip-FPGA-Tools-Setup). 
+
+## Usage
+
+```
+python3 build-bitstream.py <YAML Configuration File>
+```
+
+For example, the following command will be build the default beagleV Fire configuration:
+```
+python3 build-bitstream.py ./build-options/release/default.yaml
+```
+
+
+### YAML Configuration Files
+The YAML configuration files are located in the "build-options" directory.
+
+| Configuration File | Description                                                |
+| ------------------ | ---------------------------------------------------------- |
+| default.yaml       | Default gateware including default cape and M.2 interface. |
+| minimal.yaml       | Minimal Linux system including Ethernet. No FPGA gateware. |
+| robotics.yaml      | Similar to default but supporting the Robotics cape.       |
+
+## Supported Platforms
+The BeagleV Fire gateware builder has been tested on Ubuntu 20.04.