Added readme for AMO (compliance stuff)

README-AMO.md

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+# Build guide for AMO
+
+## Build platform
+
+The extension is built on a PC running Manjaro Linux. Yarn is the package manager of choice. 
+
+Yarn version: 1.16.0
+Node version: v11.15.0
+
+
+## Installing dependencies
+
+`yarn install --frozen-lockfile`
+
+According to Yarn documentation, that should install the exact version of dependencies that the extension is using.
+
+## Reproducing build
+
+`npm run build`
+
+The compiled code pops up in /dist.