Add building and clangd instructions

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+# Building and flashing
+
+1. First, consult the ESP-IDF docs for the basic toolchain setup:
+https://docs.espressif.com/projects/esp-idf/en/latest/esp32/get-started/linux-macos-setup.html
+2. Once you've done this, you should be able to build with `idy.py build`
+3. TODO: Verify flashing configuration once we finally get our modules
+
+# clangd setup
+
+A regular build will generate `build/compile_commands.json`, which clangd will
+automatically pick up. However, there are a couple of additional steps to get
+everything to place nicely.
+
+First, you will need to download the xtensa clang toolchain. You can do this
+via ESP-IDF by running  `idf_tools.py install xtensa-clang`
+
+This will install their prebuild clang into a path like `~/.espressif/tools/xtensa-clang/VERSION/xtensa-esp32-elf-clang/`
+
+Next, you will need to configure clangd to use this version of clang, plus
+forcible remove a couple of GCC-specific build flags. Do this by creating
+`.clangd` in the root directory of your project, with contents like so:
+
+```
+CompileFlags:
+  Add: [-mlong-calls, -isysroot=/Users/YOU/.espressif/tools/xtensa-clang/VERSION/xtensa-esp32-elf-clang]
+  Remove: [-fno-tree-switch-conversion, -mtext-section-literals, -mlongcalls, -fstrict-volatile-bitfields]
+```
+
+You should then get proper LSP integration via clangd, give or take a couple of
+oddities (e.g. for some reason, my install still can't see `stdio.h`. NBD tho.)
+
+Expect this integration to improve sometime in the near future, per [this forum
+thread](https://esp32.com/viewtopic.php?f=13&t=29563).