Create README.md for atlas metadata files

Add README.md with metadata file descriptions for atlas.

Author: gaow

data/atlas_metadata/README.md

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+5 Metadata files used to generate [this table](https://github.com/StatFunGen/xqtl-resources/blob/main/data/genes/unified_AD_loci_xQTL_summary.xlsx):
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+- `metadata_analysis.csv` containing all exported tables paths
+- `contexts_metadata.csv` containing contexts (datasets) label mapping between the different analysis/exported tables
+- `columns_metadata.tsv` containing all columns to keep for the excel sheets, with associated metadata (column width, coloring..)
+- `excel_metadata.tsv` containing some meta information for the excel sheet construction
+- `pattern_coloring.tsv` containing all patterns to color in the excel sheet
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+1 metadata optional:
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+- `long_table_columns_selection.csv` to generate a long table with selected columns from the table res_allanalysis_ADloci_overlap.csv.gz generated in step III  (in this table, each row is a variant-ADlocus-Method-context-gene_name information, and so facilitate querying informations, see also [this file](https://github.com/StatFunGen/xqtl-paper/blob/main/main_text/6_AD_xQTL_genes/staging/gene_priorization_table/complete_ADlocus_level_summary.R))