Porvide instructions how to build

[georgik]

Current version of README does not contain instructions on how to build the project. Build fails with:

error: environment variable `SSID` not defined at compile time
  --> src/main.rs:49:20
   |
49 | const SSID: &str = env!("SSID");
   |                    ^^^^^^^^^^^^
   |
   = help: use `std::env::var("SSID")` to read the variable at run time
   = note: this error originates in the macro `env` (in Nightly builds, run with -Z macro-backtrace for more info)

error: environment variable `PASSWORD` not defined at compile time
  --> src/main.rs:50:24
   |
50 | const PASSWORD: &str = env!("PASSWORD");
   |                        ^^^^^^^^^^^^^^^^
   |
   = help: use `std::env::var("PASSWORD")` to read the variable at run time
   = note: this error originates in the macro `env` (in Nightly builds, run with -Z macro-backtrace for more info)

[JurajSadel]

Thank you @georgik! Added in https://github.com/JurajSadel/esp32c3-no-std-async-mqtt-demo/commit/710f74d0d48eda6803f5a804d6ab73cfcb02019c