[librsvg/hospitality] CONTRIBUTING.md: Note on adding tests

[Federico Mena Quintero <federico src gnome org>]

commit 788cd43dbbcae08adea875e7e01bbdef62173348
Author: Federico Mena Quintero <federico gnome org>
Date:   Thu Jan 18 20:55:18 2018 -0600

    CONTRIBUTING.md: Note on adding tests
    
    tests/README.md: Note on unit tests vs. black box tests

 CONTRIBUTING.md |  6 ++++++
 tests/README.md | 23 +++++++++++++++++++++++
 2 files changed, 29 insertions(+)
---
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 8e5bd41..4bc99b7 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -121,6 +121,11 @@ libtool --mode=execute ./rsvg-convert -o foo.png foo.svg
 Then you can view the resulting `foo.png` image.  Alternatively, you
 can use `./rsvg-view-3` for a quick-and-dirty SVG viewer.
 
+**Please update the test suite** with a suitable example file once you
+have things working (or before even writing code, if you like
+test-driven development), so we can avoid regressions later.  The test
+suite is documented in [`tests/README.md`][tests-readme].
+
 [coc]: code_of_conduct.md
 [gitlab]: https://gitlab.gnome.org/GNOME/librsvg
 [bugs-browse]: https://gitlab.gnome.org/GNOME/librsvg/issues
@@ -128,3 +133,4 @@ can use `./rsvg-view-3` for a quick-and-dirty SVG viewer.
 [tests-readme]: tests/README.md
 [blog]: https://people.gnome.org/~federico/blog/librsvg-build-infrastructure.html
 [toplevel-makefile]: Makefile.am
+[tests-readme]: tests/README.md
diff --git a/tests/README.md b/tests/README.md
index 622f52b..533048b 100644
--- a/tests/README.md
+++ b/tests/README.md
@@ -30,9 +30,32 @@ These are all "black box tests": they run the library with its public
 API, and test the results.  They do not test the library's internals;
 just the output.
 
+
+Unit tests
+----------
+
 Additionally, the library's source code has smaller unit tests for
 particular sections of the code.
 
+**It is better to catch errors early**, in the unit tests, if
+possible.  The test suite in this directory is for black box tests,
+which run the library as a normal program would use it.
+
+* **What should be in a unit test** - a small test of an algorithm; a
+  check for computed values given some starting values; checks for
+  edge cases.
+
+* **What should be in these black-box tests** - rendering tests that
+  exercise a particular part of the code; CSS cascading tests; images
+  that expose bugs and that we want to avoid regressing on later.
+
+For example, there are unit tests of the path data parser (the `<path
+d="M10 10 L20 20 ...">` element and its `d` attribute, to ensure that
+the parser handles all the path commands and catches errors
+appropriately.  Correspondingly, there are a bunch of black-box tests
+that exercise particular features of path rendering ("does this
+actually draw a line, or an arc?").
+
 
 Running the test suite
 ----------------------