Update README.md to include workaround for processing gVCFs.

Change-Id: I0985dffc7f83b4c4076063521ec1ba983a4b817d

Author: cmclean

README.md

@@ -134,3 +134,7 @@ unambiguous mapping. The guarantee GenomeWarp provides is that all
 confidently-called regions in the target assembly faithfully reproduce the same
 haplotypes as were provided in the query assembly (i.e., GenomeWarp gives 100%
 specificity at a possible sacrifice to sensitivity).
+
+GenomeWarp currently handles variant-only VCF files (i.e. gVCFs are not
+supported). A gVCF can be processed using the workaround [described
+here](https://github.com/verilylifesciences/genomewarp/issues/2).