BIP 21: Clarify case-sensitivity

The "bitcoin:" URI scheme needs to be case-insensitive according to the RFC,
but the query parameters should be case-sensitive according to the bitcoin-qt
client implementation.

bip-0021.mediawiki

@@ -48,6 +48,8 @@ Elements of the query component may contain characters outside the valid range.
 
 Here, "qchar" corresponds to valid characters of an RFC 3986 URI query component, excluding the "=" and "&" characters, which this BIP takes as separators.
 
+The scheme component ("bitcoin:") is case-insensitive, and implementations must accept any combination of uppercase and lowercase letters. The rest of the URI is case-sensitive, including the query parameter keys.
+
 === Query Keys ===
 
 *label: Label for that address (e.g. name of receiver)