Improve README - link to gerrit, describe vim2 build process

Change-Id: Ie340ae7a9ff0675a48c6ef8996cef7e0c32bcde3

README.md

diff --git a/README.md b/README.md
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--- a/README.md
+++ b/README.md
@@ -1,9 +1,40 @@
 # SPDX-License-Identifier:	GPL-2.0+
 # u-boot
 
-This is a version of u-boot, modified to support Fuchsia on the Khadas vim2
-and NXP imx8mevk boards.
+This is a version of u-boot, modified to support Fuchsia on the Khadas vim2,
+vim3, and NXP imx8mevk boards.
 
 The master branch in this repository is empty.
 
 Please check out one of the platform specific branches, like vim2 or imx8mevk
+for access to the source.
+
+u-boot uses the Gerrit code review system to review and accept patches. Our
+instance is located at [https://third-party-mirror-review.googlesource.com/].
+Please take a look at the CONTRIBUTING file and at that link if you're
+interested.
+
+Building vim2 Bootloader
+========================
+
+To build the vim2 bootloader for Fuchsia:
+
+Prerequisites - arm and aarch64 toolchains. On Debian/Ubuntu systems, for example:
+```
+apt-get install gcc-arm-none-eabi gcc-aarch64-linux-gnu
+```
+
+1. Check out the vim2 branch
+```
+git checkout -b vim2 origin/vim2
+```
+2. Configure the target
+```
+make kvim2_defconfig
+```
+3. Build u-boot.bin
+```
+make CROSS_COMPILE=aarch64-linux-gnu-
+```
+
+You now have a u-boot.bin that you can flash to a target with using fastboot.