Add docs on how to upgrade buildroot

Author: Jason Mobarak

README.md

@@ -1 +1,50 @@
 Obfuscating compiler for ARM using https://github.com/obfuscator-llvm/obfuscator.
+
+This is designed to be integrated with buildroot in order to provide an 
+obfuscating compiler for components that need it.
+
+To integrate with with buildroot, a package.mk will look like this (the `*_SOURCE`
+var below is important, you'll need to interact with the GitHub API to find it
+when upgrading to a new release):
+
+```make
+HOST_LLVM_OBFUSCATOR_VERSION = v4
+HOST_LLVM_OBFUSCATOR_SOURCE = 5086675
+
+HOST_LLVM_OBFUSCATOR_SITE = https://$(GITHUB_TOKEN):@api.github.com/repos/swift-nav/llvm-obfuscator-arm/releases/assets
+HOST_LLVM_OBFUSCATOR_METHOD = wget
+HOST_LLVM_OBFUSCATOR_DL_OPTS = --auth-no-challenge --header='Accept:application/octet-stream'
+
+HOST_LLVM_OBFUSCATOR_ACTUAL_SOURCE_TARBALL = llvm-obfuscator-arm-$(HOST_LLVM_OBFUSCATOR_VERSION).tar.xz
+HOST_LLVM_OBFUSCATOR_DEPENDENCIES = host-xz
+
+define HOST_LLVM_OBFUSCATOR_PRE_EXTRACT_FIXUP
+	if ! [ -e $(DL_DIR)/$(HOST_LLVM_OBFUSCATOR_ACTUAL_SOURCE_TARBALL) ]; then \
+		mv -v $(DL_DIR)/$(HOST_LLVM_OBFUSCATOR_SOURCE) $(DL_DIR)/$(HOST_LLVM_OBFUSCATOR_ACTUAL_SOURCE_TARBALL); fi
+	$(eval HOST_LLVM_OBFUSCATOR_SOURCE=$(HOST_LLVM_OBFUSCATOR_ACTUAL_SOURCE_TARBALL))
+endef
+
+HOST_LLVM_OBFUSCATOR_PRE_EXTRACT_HOOKS += HOST_LLVM_OBFUSCATOR_PRE_EXTRACT_FIXUP
+
+define HOST_LLVM_OBFUSCATOR_INSTALL_CMDS
+	mkdir -p $(HOST_DIR)/opt/llvm-obfuscator
+	rsync -az $(@D)/opt/llvm-obfuscator/ $(HOST_DIR)/opt/llvm-obfuscator/
+endef
+
+LLVM_OBF_CC = $(HOST_DIR)/opt/llvm-obfuscator/wrappers/bin/arm-linux-gnueabihf-clang
+LLVM_OBF_CXX = $(HOST_DIR)/opt/llvm-obfuscator/wrappers/bin/arm-linux-gnueabihf-clang++
+
+$(eval $(host-generic-package))
+```
+
+To upgrade to a new release of the compiler, use the GitHub API to find the
+new artifact ID:
+
+```bash
+> export GITHUB_TOKEN=... # your GitHub auth token
+> curl -sSL -H "Authorization: token $GITHUB_TOKEN" -H "Accept: application/vnd.github.v3.raw" https://api.github.com/repos/swift-nav/llvm-obfuscator-arm/releases/latest | jq '.assets[0].id,.assets[0].name,.tag_name'
+
+5086675
+"llvm-obfuscator-arm.txz"
+"v4"
+```