doc: updated README & created updating guide and node >= 12

doc/updating_guide.md

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+# Updating guide
+
+## From v1.x to 2.x (or latest)
+
+Configuration handling has been updated: the ability to change settings from the CLI or the environment has been removed.
+
+**Configuration is now handled with a yml config file**, which can be overridden with the `--config` CLI argument.
+
+Here is the mapping from old cli argument / env variables to the new config file object:
+
+- accesstoken => `backend.accessToken`
+- webdav => `backend.enableWebdav`
+- disablesmallestscreen => `frontend.showSmallestScreenIndicator`