[PATCH v2 1/8] diff-highlight: mention build instructions

[Jeff King <peff@peff.net>]

Once upon a time, this was just a script in a directory that could be
run directly. That changed in 0c977dbc81 (diff-highlight: split code
into module, 2017-06-15). Let's update the README to make it more clear
that you need to run make.

Signed-off-by: Jeff King <peff@peff.net>
---
 contrib/diff-highlight/README | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/contrib/diff-highlight/README b/contrib/diff-highlight/README
index 1db4440e68..9c89146fb0 100644
--- a/contrib/diff-highlight/README
+++ b/contrib/diff-highlight/README
@@ -39,10 +39,21 @@ visually distracting.  Non-diff lines and existing diff coloration is
 preserved; the intent is that the output should look exactly the same as
 the input, except for the occasional highlight.
 
+Build/Install
+-------------
+
+You can build the `diff-highlight` script by running `make` from within
+the diff-highlight directory. There is no `make install` target; you can
+copy the built script to your $PATH.
+
+You can run diff-highlight's internal tests by running `make test`. Note
+that you must also build Git itself first (by running `make` from the
+top-level of the project).
+
 Use
 ---
 
-You can try out the diff-highlight program with:
+You can try out the built diff-highlight program with:
 
 ---------------------------------------------
 git log -p --color | /path/to/diff-highlight
-- 
2.53.0.1051.ga14e96f895