i.atcorr: +README to document the sources of spectral responses

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@72120 15284696-431f-4ddb-bdfa-cd5b030d7da7

imagery/i.atcorr/sensors_csv/README

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+AVNIR
+=====
+  see ?
+  files
+    AVNIR.csv
+
+Landsat
+=======
+ see https://landsat.usgs.gov/spectral-characteristics-viewer
+ files
+   etmplus.csv
+   landsat_8.csv
+
+geoeye
+======
+  see https://apollomapping.com/blog/new-documents-atmospheric-correction-digitalglobe-satellites-2
+  files
+    geoeye1.csv
+
+ikonos
+======
+  see ?
+  files
+    ikonos.csv
+
+pleiades
+========
+  see ?
+  files
+    pleiades1a.csv
+    pleiades1b.csv
+
+PRISM
+=====
+  see ?
+  files
+    PRISM-B.csv
+    PRISM-F.csv
+    PRISM-N.csv
+
+quickbird
+=========
+  see https://apollomapping.com/blog/new-documents-atmospheric-correction-digitalglobe-satellites-2
+  files
+    quickbird2.csv
+
+rapideye
+========
+  see https://resa.planet.com/files/2014-06/Spectral_Response_Curves.pdf
+  files
+    rapideye.csv
+
+sentinel
+========
+  see https://sentinels.copernicus.eu/documents/247904/685211/S2-SRF_COPE-GSEG-EOPG-TN-15-0007_3.0.xlsx
+    updates on https://sentinels.copernicus.eu/web/sentinel/home
+    go to Home › User Guides › Sentinel-2 MSI › S2-MSI Document Library
+  files
+    sentinel_2A_msi.csv
+
+spot
+====
+  see ?
+  files
+    spot6.csv
+    spot7.csv
+    VGT1_spot4.csv
+    VGT2_spot5.csv
+
+worldview2
+=========
+  see https://apollomapping.com/blog/new-documents-atmospheric-correction-digitalglobe-satellites-2
+  files
+    worldview2.csv
+
+worldview3
+=========
+  see ?
+  files
+    worldview3.csv