Update README.md instructions

Author: Pedro H

README.md

@@ -9,5 +9,6 @@ Unique and original calendar/task organizer spreadsheet template.
 
 ## How to build
 ### Google Sheets
-### LibreOffice Calc
-### Microsoft Excel
+1. `git clone https://github.com/DevWannabe-dot/sheets-calendar-apps-script.git`
+2. `.\sheets-calendar-apps-script\sheets\Calendario-1.0.url`
+3. Proceed to `Extensions > Apps Script` to edit the source files at `.\sheets-calendar-apps-script\src\sheets`