Help users download from custom sources

Describe in the README how to support downloading from a custom source, and
add a source code location for examples.

1 files changed, 26 insertions, 0 deletions

diff --git a/README.md b/README.md
index f626dd1..a0aebad 100644
--- a/README.md
+++ b/README.md
@@ -105,6 +105,32 @@ chmod +x /usr/local/sbin/install-kernel
 
 ## Source
 
+### Customizing Update Downloads
+
+You can add a section to the [script](src/slack-autoupdate) to pull updates
+using different tools or sources.  The sections use the following structure,
+and some examples are provided in the [recipes](src/recipes) directory.
+
+```
+if ! OUTPUT="$(
+  # Redirect error output to standard output.
+  exec 2>&1
+
+  # Download updates into $STAGING_DIR.
+  #
+  # On failure the command output is appended to the error file.
+  # Update information should be appended to the $UPDATE_INFO file.
+
+)"; then
+  if [ -f "$UPDATE_ERROR" ]; then
+    >>"$UPDATE_ERROR" echo ""
+    >>"$UPDATE_ERROR" echo ""
+  fi
+
+  >>"$UPDATE_ERROR" echo -e "[Update Name]:\n\n$OUTPUT"
+fi
+```
+
 ### Testing
 
 You can test the project by running the shell files under the 'test' folder.