docs: describe the contents of vlib/v/gen/c/testdata/ and how to write tests for the checker, that still do not have good codegen counterpart

vlib/v/gen/c/testdata/README.md

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+## Purpose: tests data for the output of V's C code generator
+
+## TLDR: `v vlib/v/gen/c/coutput_test.v`
+
+coutput_test.v is a *test runner*, that checks whether the generated C 
+source code matches *all* expectations, specified in *.c.must_have files,
+in the folder vlib/v/gen/c/testdata/ .
+
+Each `.c.must_have` file, *has* to have a corresponding `.vv` file.
+
+Each `.c.must_have` file, consists of multiple lines. Each of these
+lines, *should* be present *at least once* in the output, when the .vv
+file is compiled with `-o -` .
+
+Note: this test runner can also be used to verify and keep from regressing
+*partial fixes* to V's checker, i.e. ones that make a program type check fine,
+but that still lead to codegen problems. To do that, put the V code that the
+checker can typecheck in a `.vv` file in this folder, then create a
+corresponding `.c.must_have` file in this folder too. It should have a single
+line in it: `// THE END.` . The V codegen will end every source that it
+generates with that line, so its presence, effectively guarantees that cgen
+succeeded/ended, even if the produced C code can not be compiled yet.