ICU-20953 Tell GitHub to treat JSON as JSONC to fix syntax highlighting for comments.

.gitattributes

diff --git a/.gitattributes b/.gitattributes
index 83661e6..cb4a55d 100644
--- a/.gitattributes
+++ b/.gitattributes
@@ -84,3 +84,7 @@
 
 # Enable syntax highlighting on GitHub.com
 .cpyskip.txt linguist-language=Ignore-List
+
+# Use JSONC for syntax highlighting on GitHub.com
+*.json linguist-language=jsonc
+