Docs for lib builder (#3415)

* Docs for lib builder

* Typos and such

docs/esp-idf_component.md

@@ -1,6 +1,10 @@
 To use as a component of ESP-IDF
 =================================================
 
+## esp32-arduino-lib-builder
+
+For a simplified method, see [lib-builder](lib_builder.md)
+
 ## Installation
 
 - Download and install [esp-idf](https://github.com/espressif/esp-idf)

docs/lib_builder.md

@@ -0,0 +1,14 @@
+## Using esp32-arduino-lib-builder to compile custom libraries
+
+Espressif has provided a [tool](https://github.com/espressif/esp32-arduino-lib-builder) to simplify building your own compiled libraries for use in Arduino IDE (or your favorite IDE).
+To use it to generate custom libraries, follow these steps:
+1. `git clone https://github.com/espressif/esp32-arduino-lib-builder`
+2. `cd esp32-arduino-lib-builder`
+3. `./tools/update-components.sh`
+4. `./tools/install-esp-idf.sh` (if you already have an $IDF_PATH defined, it will use your local copy of the repository)
+5. `make menuconfig` or directly edit sdkconfig.
+6. `./build.sh`
+
+The script automates the process of building [arduino as an ESP-IDF component](https://github.com/espressif/arduino-esp32/blob/master/docs/esp-idf_component.md).
+Once it is complete, you can cherry pick the needed libraries from `out/tools/sdk/lib`, or run `tools/copy-to-arduino.sh` to copy the entire built system.
+`tools/config.sh` contains a number of variables that control the process, particularly the $IDF_BRANCH variable.  You can adjust this to try building against newer versions, but there are absolutely no guarantees that any components will work or even successfully compile against a newer IDF.