doc: add BUILD.md

Author: qzhuyan

BUILD.ubuntu20.04.md

@@ -0,0 +1,28 @@
+# BUILD on ubuntu 20.04
+
+## Install build chain
+
+```sh
+sudo apt-add-repository ppa:lttng/stable-2.13
+sudo apt-get update
+sudo apt-get install -y lttng-tools lttng-modules-dkms babeltrace liblttng-ust-dev build-essential cmake
+```
+
+## Install OTP 
+```
+ wget https://packages.erlang-solutions.com/erlang/debian/pool/esl-erlang_24.3.3-1~ubuntu~focal_amd64.deb
+ sudo apt install  ./esl-erlang_24.3.3-1~ubuntu~focal_amd64.deb
+```
+
+## Fetch rebar3
+
+``` bash
+wget https://s3.amazonaws.com/rebar3/rebar3 && chmod +x rebar3
+export PATH=$PATH:./
+```
+
+## then 
+
+```bash
+make
+```