docs: add docs/README.md with headline

docs/README.md

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+# Data Entry
+
+The Data Entry app is where you manually enter aggregated data in DHIS2. You
+register data for an organisation unit, a period, and a set of data elements
+(data set) at a time. A data set often corresponds to a paper-based data
+collection tool. You configure the data sets in the Maintenance app.
+
+**Note:**
+
+If a data set has both a section form and a custom form, the system displays
+the custom form during data entry. Users who enter data can't select which form
+they want to use. In web-based data entry the order of display preference is:
+
+1. Custom form (if it exists)
+1. Section form (if it exists)
+1. Default form
+
+<!-- @TODO: We don't support mobile browser currently, is this something we
+need to support for feature parity? -->
+Mobile devices do not support custom forms. In mobile-based data entry the
+order of display preference is:
+
+1. Section form (if it exists)
+1. Default form
+
+## Editing forms in an auto-generated form
+### Tabbed forms
+## Editing forms in a custom form
+## Data values
+### Marking data values for follow-up
+### Add/Edit min- and max-range of the value
+## Offline support
+### Editing approved forms
+## Related resources