Add links to files used by python scripts.

Closes #2150

src/gen-arabic-table.py

@@ -1,5 +1,10 @@
 #!/usr/bin/env python
 
+# Input files:
+# - https://unicode.org/Public/12.0.0/ucd/ArabicShaping.txt
+# - https://unicode.org/Public/12.0.0/ucd/UnicodeData.txt
+# - https://unicode.org/Public/12.0.0/ucd/Blocks.txt
+
 from __future__ import print_function, division, absolute_import
 
 import io, os.path, sys

src/gen-emoji-table.py

@@ -1,5 +1,7 @@
 #!/usr/bin/python
 
+# Input file: https://www.unicode.org/Public/emoji/12.0/emoji-data.txt
+
 from __future__ import print_function, division, absolute_import
 import sys
 import os.path

src/gen-indic-table.py

@@ -1,5 +1,10 @@
 #!/usr/bin/env python
 
+# Input files:
+# - https://unicode.org/Public/12.0.0/ucd/IndicSyllabicCategory.txt
+# - https://unicode.org/Public/12.0.0/ucd/IndicPositionalCategory.txt
+# - https://unicode.org/Public/12.0.0/ucd/Blocks.txt
+
 from __future__ import print_function, division, absolute_import
 
 import io, sys

src/gen-tag-table.py

@@ -16,6 +16,10 @@ back to BCP 47 tags. Ambiguous OpenType tags (those that correspond to
 multiple BCP 47 tags) are listed here, except when the alphabetically
 first BCP 47 tag happens to be the chosen disambiguated tag. In that
 case, the fallback behavior will choose the right tag anyway.
+
+Input files:
+- https://docs.microsoft.com/en-us/typography/opentype/spec/languagetags
+- https://www.iana.org/assignments/language-subtag-registry/language-subtag-registry
 """
 
 from __future__ import absolute_import, division, print_function, unicode_literals

src/gen-ucd-table.py

@@ -1,5 +1,7 @@
 #!/usr/bin/env python
 
+# Input file: http://unicode.org/Public/UCD/latest/ucdxml/ucd.nounihan.grouped.zip
+
 from __future__ import print_function, division, absolute_import
 
 import io, os.path, sys, re

src/gen-use-table.py

@@ -1,6 +1,12 @@
 #!/usr/bin/env python
 # flake8: noqa
 
+# Input files:
+# - https://unicode.org/Public/12.0.0/ucd/IndicSyllabicCategory.txt
+# - https://unicode.org/Public/12.0.0/ucd/IndicPositionalCategory.txt
+# - https://unicode.org/Public/12.0.0/ucd/UnicodeData.txt
+# - https://unicode.org/Public/12.0.0/ucd/Blocks.txt
+
 from __future__ import print_function, division, absolute_import
 
 import io

src/gen-vowel-constraints.py

@@ -6,6 +6,8 @@ It creates ``_hb_preprocess_text_vowel_constraints``, which inserts dotted
 circles into sequences prohibited by the USE script development spec.
 This function should be used as the ``preprocess_text`` of an
 ``hb_ot_complex_shaper_t``.
+
+Input file: https://unicode.org/Public/12.0.0/ucd/Scripts.txt
 """
 
 from __future__ import absolute_import, division, print_function, unicode_literals