Suggestion for README

[billmetz]

This is a great project.
A small nitpick; it's not immediately obvious how it is to be used, perhaps a change such as the following would help other users -

`--- README.md.orig 2020-01-05 11:32:18.609521000 +1100
+++ README.md 2020-01-05 11:30:33.173459773 +1100
@@ -7,8 +7,10 @@
make esp32

-Uses [pfalcon's esp-open-sdk](https://github.com/pfalcon/esp-open-sdk) for
+The script automatically downloads and uses
+[pfalcon's esp-open-sdk](https://github.com/pfalcon/esp-open-sdk) for
the esp8266 toolchain (in toolchain-only mode), and the [official Espressif
repo](https://github.com/espressif/crosstool-NG) for the esp32 toolchain.
+See those two projects for advice on dependencies and installation.

Based on the [nodemcu-prebuilt-toolchains](https://github.com/jmattsson/nodemcu-prebuilt-toolchains) script.
`