Mention BibParser in README

[blegat]

It would be helpful to mention the alternative https://github.com/Humans-of-Julia/BibParser.jl in the README.
The line

BibTeX.jl/src/BibTeX.jl

Line 17 in 08bd44b

tokens = collect(m.match for m in eachmatch(r"[^\s\n\"#{}@,=]+|\n|\"|#|{|}|@|,|=", text))

adds spaces around { and } which makes it tricky to use for. I spent some time trying to figure out a workaround until I found BibParser which seems more feature-rich, actively maintained and the one used internally by https://github.com/JuliaDocs/DocumenterCitations.jl.