[tracker/sam/debug: 4/4] HACKING.md: Document the available debug options

[Sam Thursfield <sthursfield src gnome org>]

commit 947719f351eb2f4b6a1b886c38c8f086f15727a6
Author: Sam Thursfield <sam afuera me uk>
Date:   Mon Apr 20 02:44:51 2020 +0200

    HACKING.md: Document the available debug options

 HACKING.md | 7 +++++++
 1 file changed, 7 insertions(+)
---
diff --git a/HACKING.md b/HACKING.md
index 61a40e192..7faaef41f 100644
--- a/HACKING.md
+++ b/HACKING.md
@@ -37,6 +37,13 @@ The following environment variables control logging from Tracker daemons:
   * `TRACKER_DEBUG`: takes a comma-separated list of keywords to enable
     extra debugging output. Use the keyword 'help' for a list of keywords.
 
+    Options for libtracker-sparql include:
+
+        - ontology-changes
+        - sql-statements
+
+    See the relevant `man` page for options relevant to tracker-miner-fs.
+
 You can set these variables when using `tracker-sandbox`, and when running the
 Tracker test suite. Note that Meson will not print log output from tests by
 default, use `meson test --verbose` or `meson test --print-errorlogs` to