This PR makes some minor clarifications to the annotaTR readme.

• Adds a note that older bcftools versions might switch allele order when doing bcftools merge, and that we tested on v1.20
• Minor clarification to options for --match-refpanel-on.

Resolves #244

Checklist

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• [x ] I've added tests for any new functionality. Or, if this PR fixes a bug, I've added test(s) that replicate it
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