Resolve "ELN tests, documentation, and examples"

Closes #796 (closed)

nomad/datamodel/metainfo/eln/material_library/README.md

+# Material Library
+
+This an example ELN schema inspired by a lab working with thin-file material library samples.
+The schema has three top-level entry types (i.e. sections that extend `EntryData`):
+sample, chemical, instrument. It shows some of NOMAD's ELN capabilities:
+
+- References to files: Chemicals can have safety sheet PDFs, Instruments refer to calibration documentation.
+- References between different entry types: Sample processes can refer to chemicals and
+instruments.
+- Custom schema specific processing of referenced files: The PVD Evaporation process
+can reference a `.csv` file that is parsed and populates values of the respective section.
+The XRD measurement can reference a `.xrdml` file which is parsed with the xrdtools library.
+- A boolean flag on processes `creates_layer` automatically creates a new layer sub-section
+in the sample and links it back the the process that *created* it.
+
+