Document the Ubuntu 20.04 socat bug (unikraft#29) in README.

Signed-off-by: Xiangyi Meng <xymeng16@gmail.com>

README.md

@@ -14,6 +14,8 @@ install the requirements:
 
     apt-get install -y --no-install-recommends build-essential libncurses-dev libyaml-dev flex git wget socat bison unzip uuid-runtime;
 
+Note: Ubuntu 20.04 users may suffer from issue [#29](https://github.com/unikraft/kraft/issues/29) due to this [bug](https://bugs.launchpad.net/ubuntu/+source/socat/+bug/1883957) of `socat-1.7.3.3`. If you are using Ubuntu 20.04, please make sure to compile and install the latest version of `socat` retrieved from [this page](http://www.dest-unreach.org/socat/download/).
+
 To install `kraft` simply run:
 
     pip3 install git+https://github.com/unikraft/kraft.git@staging
@@ -30,23 +32,23 @@ is by running the following commands:
 
 At present, Unikraft and kraft support the following applications:
 
-* [C "hello world"](https://github.com/unikraft/app-helloworld) (`helloworld`);
-* [C "http reply"](https://github.com/unikraft/app-httpreply) (`httpreply`);
-* [C++ "hello world"](https://github.com/unikraft/app-helloworld-cpp) (`helloworld-cpp`);
-* [Golang](https://github.com/unikraft/app-helloworld-go) (`helloworld-go`);
-* [Python 3](https://github.com/unikraft/app-python3) (`python3`);
-* [Micropython](https://github.com/unikraft/app-micropython) (`micropython`);
-* [Ruby](https://github.com/unikraft/app-ruby) (`ruby`);
-* [Lua](https://github.com/unikraft/app-lua) (`lua`);
-* [Click Modular Router](https://github.com/unikraft/app-click) (`click`);
-* [JavaScript (Duktape)](https://github.com/unikraft/app-duktape) (`duktape`);
-* [Web Assembly Micro Runtime (WAMR)](https://github.com/unikraft/app-wamr) (`wamr`);
-* [Redis](https://github.com/unikraft/app-redis) (`redis`);
-* [Nginx](https://github.com/unikraft/app-nginx) (`nginx`);
-* [SQLite](https://github.com/unikraft/app-sqlite) (`sqlite`);
+- [C "hello world"](https://github.com/unikraft/app-helloworld) (`helloworld`);
+- [C "http reply"](https://github.com/unikraft/app-httpreply) (`httpreply`);
+- [C++ "hello world"](https://github.com/unikraft/app-helloworld-cpp) (`helloworld-cpp`);
+- [Golang](https://github.com/unikraft/app-helloworld-go) (`helloworld-go`);
+- [Python 3](https://github.com/unikraft/app-python3) (`python3`);
+- [Micropython](https://github.com/unikraft/app-micropython) (`micropython`);
+- [Ruby](https://github.com/unikraft/app-ruby) (`ruby`);
+- [Lua](https://github.com/unikraft/app-lua) (`lua`);
+- [Click Modular Router](https://github.com/unikraft/app-click) (`click`);
+- [JavaScript (Duktape)](https://github.com/unikraft/app-duktape) (`duktape`);
+- [Web Assembly Micro Runtime (WAMR)](https://github.com/unikraft/app-wamr) (`wamr`);
+- [Redis](https://github.com/unikraft/app-redis) (`redis`);
+- [Nginx](https://github.com/unikraft/app-nginx) (`nginx`);
+- [SQLite](https://github.com/unikraft/app-sqlite) (`sqlite`);
 
 For more information about that command type `kraft up -h`. For more information
-about `kraft` type ```kraft -h``` or read the documentation at
+about `kraft` type `kraft -h` or read the documentation at
 [Unikraft's website](https://docs.unikraft.org). If you find any problems please
 [fill out an issue](https://github.com/unikraft/tools/issues/new/choose). Thank
 you!