Add vendor files for go build

vendor/github.com/alecthomas/chroma/lexers/README.md

@@ -0,0 +1,19 @@
+# Lexer tests
+
+The tests in this directory feed a known input `testdata/<name>.actual` into the parser for `<name>` and check
+that its output matches `<name>.exported`.
+
+## Running the tests
+
+Run the tests as normal:
+```go
+go test ./lexers
+```
+
+## Updating the existing tests
+
+You can regenerate all the test outputs
+
+```go
+RECORD=true go test ./lexers
+```