adding samtools spec

Author: linsalrob

Workshops/INRB2023.md

@@ -127,6 +127,8 @@ minimap2 --split-prefix=tmp$$ -a -xsr GCA_000001405.15_GRCh38_no_alt_plus_hs38d1
 samtools index 788707_20180129.bam
 ```
 
+Here is the [samtools specification](https://samtools.github.io/hts-specs/SAMv1.pdf), and the description of the columns is on page 6.
+
 Now, we use `samtools` flags to filter out the human and not human sequences. You can find out what the flags mean using the [samtools flag explainer](https://broadinstitute.github.io/picard/explain-flags.html)
 
 ### human only sequences