link to esphome added

README.md

@@ -19,6 +19,7 @@ higher concentrations of CO2 inside will make you sleepy and have an impact on y
 ## build
 
 just solder everything according to the connection schema.
+get yourself a copy of esphome (https://esphome.io/) and compile the firmware with 'esphome co2sensor.yaml run' - hold boot on your esp until fw upload start.
 
 ### schema
 
@@ -66,4 +67,4 @@ either way it might be a good idea to have a capacitor between VCC and GND. my p
  * cooldown time for buzzer (alarm only once every n minutes) -> lambda/interval/on_time?
  * add openings for mh-z19 to the case
  * adjust light tunnel to use frosted acrylic glass
- * add a silent switch for meeting rooms and such (no buzzer, but flash at a regular interval to attract attention)
+ * add a silent switch for meeting rooms and such (no buzzer, but flash at a regular interval to attract attention)