Add BOOLEAN to "A Warm Welcome to ASN.1 and DER"

[aarongable]

We skip over booleans in our description of ANS.1's basic types, in our table of tag values, and in our description of DER serialization. This seems like an odd omission, given that they're commonly used in x509 (e.g. for the Critical field on extensions) and that DER places interesting restrictions on how they're encoded. Let's add a brief treatment of bools to this doc.