Add additional instructions to source build (#137)

* Add additional instructions to source build

I needed to upgrade pip and wheel before I was able to build from source

* Update README.md

README.md

@@ -34,7 +34,11 @@ The full-featured ESP8266 flashing tool for [Tasmota](https://github.com/arendst
  Install tasmotizer: `pip3 install tasmotizer`   
  Then simply run `tasmotizer.py` or `python3 -m tasmotizer.py` from the shell/command line
 
- - Option 3: Clone the repo, `pip3 install -r requirements.txt` followed by `python3 tasmotizer.py ` and flash away!
+ - Option 3: Build from source<br/>
+ Clone the repo<br/>
+ Upgrade pip and wheel packages: `pip3 install --upgrade pip wheel`<br/>
+ Install requirements `pip3 install -r requirements.txt`<br/>
+ Run `python3 tasmotizer.py ` and flash away!
 
 ## Screenshots