[test] Add README about how to add shaping tests

test/shaping/Makefile.am

@@ -14,6 +14,7 @@ manifests:
 	@$(srcdir)/hb-manifest-update "$(srcdir)/texts" "$(srcdir)/fonts" "$(srcdir)/tests"
 
 EXTRA_DIST += \
+	README.md \
 	hb-diff \
 	hb-diff-colorize \
 	hb-diff-filter-failures \

test/shaping/README.md

+Adding tests
+============
+
+You can test shaping of a unicode sequence against a font like this:
+```sh
+$ ./hb-unicode-encode 41 42 43 627 | ../../util/hb-shape font.ttf
+```
+assuming an in-tree build.  The 41 42 43 627 here is a sequence of
+Unicode codepoints: U+0041,0042,0043,0627.  When you are happy with
+the shape results, you can use the `record-test.sh` script to add
+this to the test suite.  `record-test.sh` requires `pyftsubset` to
+be installed.  You can get `pyftsubset` by installing
+FontTools from <https://github.com/behdad/fonttools>.
+
+To use `record-test.sh`, just put it right before the `hb-shape` invocation:
+```sh
+$ ./hb-unicode-encode 41 42 43 627 | ./record-it.sh ../../util/hb-shape font.ttf
+```
+what this does is:
+  * Subset the font for the sequence of Unicode characters requested,
+  * Compare the `hb-shape` output of the original font versus the subset
+    font for the input sequence,
+  * If the outputs differ, perhaps it is because the font does not have
+    glyph names; it then compares the output of `hb-view` for both fonts.
+  * If the outputs differ, recording fails.  Otherwise, it will move the
+    subset font file into `fonts/sha1sum` and name it after its hash,
+    and prints out the test case input, which you can then redirect to
+    an existing or new test file in `tests`, eg.:
+```sh
+$ ./hb-unicode-encode 41 42 43 627 | ./record-it.sh ../../util/hb-shape font.ttf >> tests/test-name.test
+```
+
+If you created a new test file, add it to Makefile.am so it is run.
+Check that `make test` does indeed run it, and that the test passes.
+When everything looks good, `git add` the new font as well as new
+test file if you created any.  You can see what new files are there
+by running `git status tests fonts/sha1sum`.  And commit!