Update README.md with vim3 instructions

The instructions for vim3 has been tested and verified to work by
flashing on a physical board.

README.md

diff --git a/README.md b/README.md
index 1779f85..5faae4b 100644
--- a/README.md
+++ b/README.md
@@ -14,8 +14,29 @@
 Please take a look at the CONTRIBUTING file and at that link if you're
 interested.
 
-Building vim2 Bootloader
-========================
+Building the vim3 bootloader
+============================
+
+To build the vim3 bootloader for Fuchsia:
+
+Prerequisites - arm and aarch64 toolchains. On Debian/Ubuntu systems, for example:
+```
+apt install gcc-arm-none-eabi gcc-aarch64-linux-gnu
+```
+
+1. Check out the vim3 branch
+```
+git checkout --track remotes/origin/vim3
+```
+2. Configure and build the bootloader by executing the helper script
+```
+./build_uboot_vim3_zircon.sh
+```
+
+You now have a build/u-boot.bin.unsigned that you can flash to a target with using fastboot.
+
+Building the vim2 bootloader
+============================
 
 To build the vim2 bootloader for Fuchsia: