docs: note about string versus boolean values for certain deme names.

[grahamgower]

Related to #162, we should note in the docs that certain deme names are likely to be misinterpreted as boolean values with YAML v1.1 parsers and should thus be quoted in the input file (or avoided). y, n, and no all seem like plausible deme names to me.

YAML v1.1, from https://yaml.org/type/bool.html

y|Y|yes|Yes|YES|n|N|no|No|NO
|true|True|TRUE|false|False|FALSE
|on|On|ON|off|Off|OFF

YAML v1.2, from https://yaml.org/spec/1.2.2/#1032-tag-resolution

true | True | TRUE | false | False | FALSE

I note also that null will not be interpreted as a string, but it seems pretty unlikely that anyone would use this as a deme name.

[jeromekelleher]

Possibly worth referring to the Norway problem for more context.

[molpopgen]

I guess I should figure out what demes-rs is using. The yaml crate isn't clear upon a quick read.