doc for on demand build with GitHub Actions

qmk_firmware/keyboards/keyball/readme.md

@@ -66,4 +66,19 @@ There are three keymaps provided at least:
 
 ## How to create your keymap
 
-(TO BE DOCUMENTED)
+1. Fork this Yowkees/keyball repository
+2. Checkout forked repository
+3. (OPTIONAL) Create a new branch
+4. Add a your keymap, or make some changes
+5. Commit changes and push it to your forked repository
+6. Open your forked repository with web browser
+7. Click and open "Actions" tab
+8. Click "Build a firmware on demand" in Workflows on left panel
+9. Press "Run workflow" button on right side, then you will see forms
+10. (OPTIONAL) Select a your working branch
+11. Select a "Keyboard" from drop-down list
+12. Enter the "keymap" you want to build
+13. Click "Run workflow"
+14. Wait a minute until the firmware build is finished
+15. Click a latest workflow run and open details
+16. Download built firmware in "Artifacts" section