Merge pull request #6210 from cstyan/lex-notes

Add a README.md to textparse package about how to update lexers.

pkg/textparse/README.md

+# Making changes to textparse lexers
+In the rare case that you need to update the textparse lexers, edit promlex.l or openmetricslex.l and then run the following command: 
+`golex -o=promlex.l.go promlex.l`
+
+Note that you need golex installed: 
+`go get -u modernc.org/golex`
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