add lambda info

lambda/README.md

@@ -0,0 +1,10 @@
+# Build the tippecanoe layer (binary)
+
+1. On an AWS Linux 2 instance or image:
+
+    sudo yum update
+    sudo yum install git clang sqlite-devel zlib-devel
+
+2. Create a ZIP archive with the `tippecanoe` binary at `bin/tippecanoe` and upload as a Lambda layer
+3. Copy [lambda_function.py](lambda_function.py) (python 3.9 runtime)
+4. set the environment variable `OUTPUT_BUCKET` to your output bucket; give lambda IAM role access to input/output buckets