Add guidance on rfc3642

[OR13]

https://datatracker.ietf.org/doc/html/rfc3642

Seems wise to refer to it, and either confirm we conform to it, or explicitly state we do not.

[OR13]

The <BIT-STRING> rule describes the GSER encoding of values of the
BIT STRING type without a named bit list.

BIT-STRING = bstring / hstring

If the number of bits in a BIT STRING value is a multiple of four the
<hstring> form of <BIT-STRING> MAY be used.  Otherwise, the <bstring>
form of <BIT-STRING> is used.  The <bstring> rule encodes each bit as
the character "0" or "1" in order from the first bit to the last bit.
The <hstring> rule encodes each group of four bits as a hexadecimal
number where the first bit is the most significant.  An odd number of
hexadecimal digits is permitted.

[OR13]

@mprorock your proposal have any alignment to this?

[OR13]

Sugegst closing this issue, we describe the bitstring in sufficient detail in the existing spec.