Added attrib type for Havana readthrough transcripts.

misc-scripts/attribute_types/attrib_type.txt

@@ -341,3 +341,6 @@
 #ÊAttributes added as gene_attribs to Ensembl genes that fall within a genomic region where a LRG record exists
 216	GeneInLRG	Gene in LRG	This gene is contained within an LRG region
 217	GeneOverlapLRG	Gene overlaps LRG	This gene is partially overlapped by a LRG region (start or end outside LRG)
+
+# Attrib type for Havana readthrough transcripts
+218	readthrough_tra	readthrough transcript	Havana readthrough transcripts