readme: add README.md

readme: update for R64, R2Pro and R3

README.md

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+# U-boot for BPI-R2/R64/R3
+
+![CI](https://github.com/frank-w/u-boot/workflows/CI/badge.svg?branch=2021-01-bpi)
+
+BPI-R2Pro is not supported due to internal changes in U-boot and no Open-Source-ATF.
+Last working Version is in Branch 2022-04-bpi
+
+## Requirements
+
+On x86/x64-host you need cross compile tools for the armhf architecture:
+```sh
+sudo apt-get install gcc-arm-linux-gnueabihf gcc-aarch64-linux-gnu libc6-armhf-cross u-boot-tools make gcc swig python-dev
+```
+
+## Issues
+- loadenv fails because of resized environment (4096 => 8188)
+  - backup your saved environment before update uboot or 
+    change back CONFIG_ENV_SIZE to SZ_4K (./build.sh soc)
+  - erase your saved environment
+
+```
+env erase
+```
+
+- no emmc-command (not needed "emmc pconf 0x48" = "mmc partconf 0 1 1 0")
+
+## Usage
+
+```sh
+  #edit build.conf to select bpi-r64/bpi-r3 if needed
+  ./build.sh importconfig
+  ./build.sh config #optional (menuconfig)
+  ./build.sh
+  ./build.sh install #write to sd-card
+  ./build.sh umount #umount automatic mounted partitions
+```