Adding documentation

Author: nsfilho

README.md

@@ -8,4 +8,19 @@ The goal of this docker image is provide a complete isolated environment for bui
 
 You can see a sample of use in github actions on [E12AIO3 Firmware](https://github.com/e12aio3)
 
-Thank you and enjoy! :enjoy:
+```yml
+name: ESP8266-RTOS-SDK Build
+
+on:
+    push:
+        branches: [master]
+
+jobs:
+    build:
+        runs-on: ubuntu-latest
+        steps:
+            - uses: actions/checkout@v2
+            - uses: nsfilho/esp8266-rtos-sdk@v2
+```
+
+Thank you and enjoy!

action.yml

@@ -10,6 +10,6 @@ inputs:
     default: "defconfig all size-components"
 runs:
   using: "docker"
-  image: "docker://nsfilho/esp8266-rtos-sdk"
+  image: "docker://nsfilho/esp8266-rtos-sdk:3.3"
   args:
     - ${{ inputs.makearg }}