Document the other package url that expressif has sometimes used

Author: hamishcoleman

Diff for: Makefile

@@ -3,15 +3,21 @@
 #
 
 SKETCH := CameraWebServer.ino
-
 BOARD := esp32:esp32:esp32cam
-PACKAGE_URL := https://dl.espressif.com/dl/package_esp32_index.json
 
 BOARD_FILENAME_SLUG := $(subst :,.,$(BOARD))
 
 all: $(SKETCH).$(BOARD_FILENAME_SLUG).bin
 
+
+PACKAGE_URL := https://dl.espressif.com/dl/package_esp32_index.json
+# Note that the package URL is sometimes documented as
+# https://raw.githubusercontent.com/espressif/arduino-esp32/gh-pages/package_esp32_index.json
+# At the time of writing these two urls resulted in byte-for-byte identical
+# files, so I used the shorter URL
 # I dont want to use curl|sh, but this is the documented install method ...
+
+
 ARDUINO_URL := https://raw.githubusercontent.com/arduino/arduino-cli/master/install.sh
 bin/arduino-cli: /usr/bin/curl
 	curl -fsSL $(ARDUINO_URL) | sh