Update Design Guidelines with character set guidance

[trehman-gsma]

Explicitly states that char sets must be UTF-8 to address issue #112

What type of PR is this?

• documentation

What this PR does / why we need it:

Explicitly states the supported character set is only UTF-8 as per #112

Which issue(s) this PR fixes:

Fixes #112

Special notes for reviewers:

Explicitly states that UTF-8 is the mandatory character set due to RFC 8259 (JSON Data Interchange Format)

JSON text exchanged between systems that are not part of a closed
   ecosystem MUST be encoded using UTF-8 [[RFC3629](https://datatracker.ietf.org/doc/html/rfc3629)].

   Previous specifications of JSON have not required the use of UTF-8
   when transmitting JSON text.  However, the vast majority of JSON-
   based software implementations have chosen to use the UTF-8 encoding,
   to the extent that it is the only encoding that achieves
   interoperability.

[PedroDiez]

LGTM