add input size tip to instructions

update instructions to include reminder about token dictionary

anndata_tokenizer (#170)

Add instructions to convert gene annotations w/ Ensembl Biomart

Add further explanation regarding input file format for transcriptome tokenizer

Update instructions in tokenizing example to clarify requirement for Ensembl ID

Update instructions in tokenizing example to clarify requirement for Ensembl ID

Add further explanation to tokenizer example script and updated tokenizer to match loompy raised error

Add example for tokenizing .loom dataset