Freshed up building instructions, move to a better-named file.

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+<!--
+Copyright 2023 jacqueline <me@jacqueline.id.au>
+
+SPDX-License-Identifier: CC0-1.0
+-->
+
+# Building and flashing
+
+1. Make sure you've got all of the submodules in this repo correctly initialised:
+
+```
+git submodule update --init --recursive
+```
+
+2. If this is your first time setting up the repo, then you will need to install
+the ESP-IDF tools. You can consult the [ESP-IDF docs](https://docs.espressif.com/projects/esp-idf/en/latest/esp32/get-started/linux-macos-setup.html)
+for more detailed instructions, but the TL;DR is that you'll want to run
+something like this:
+
+```
+./lib/esp-idf/install.sh esp32
+```
+
+3. As a final piece of setup, you will need to source the env file in this repo
+to correctly set up your environment for building.
+
+```
+. ./.env
+```
+
+There is also a `.env.fish` for fish users.
+
+4. You can now build the project using `idf.py build`. Or to flash the project
+onto your board, something like:
+
+```
+idf.py -p /dev/ttyUSB0 -b 115200 flash
+```
+
+(give or take the correct serial port)
+
+# Running tests
+
+Tests are implemented as a separate application build, located in the `test`
+directory. We use Catch2 as our test framework.
+
+To run them, navigate to the test directory, then build and flash as normal.
+Connect to your device via UART, and you will be presented with a terminal
+prompt that you may run tests from.
+
+To add new tests to a components, you must:
+ 1. Create a `test` subcomponent within that component. See `drivers/test` for
+    an example of this.
+ 2. Include the component in the test build and list of testable components, in
+    `test/CMakeLists.txt`.
+
+
+# VSCode setup
+
+When using the Espressif IDF extension, you may want to set the following in your settings.json file:
+```
+  "idf.espIdfPath": "${workspaceFolder}/lib/esp-idf",
+  "idf.espIdfPathWin": "${workspaceFolder}/lib/esp-idf"
+```
+
+# LSP (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 play 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: [
+    -ferror-limit=0,
+    -I/Users/YOU/.espressif/tools/xtensa-esp32-elf/esp-2021r2-patch5-8.4.0/xtensa-esp32-elf/include,
+    -I/Users/YOU/.espressif/tools/xtensa-esp32-elf/esp-2021r2-patch5-8.4.0/xtensa-esp32-elf/xtensa-esp32-elf/include,
+    -I/Users/YOU/.espressif/tools/xtensa-esp32-elf/esp-2021r2-patch5-8.4.0/xtensa-esp32-elf/xtensa-esp32-elf/include/c++/8.4.0,
+    -I/Users/YOU/.espressif/tools/xtensa-esp32-elf/esp-2021r2-patch5-8.4.0/xtensa-esp32-elf/xtensa-esp32-elf/include/c++/8.4.0/xtensa-esp32-elf,
+    -isysroot=/Users/YOU/.espressif/tools/xtensa-esp32-elf/esp-2021r2-patch5-8.4.0/xtensa-esp32-elf/xtensa-esp32-elf,
+  ]
+  Remove: [
+    -Wduplicated-cond,
+    -Wduplicated-branches,
+    -Wlogical-op,
+    -fno-tree-switch-conversion,
+    -mtext-section-literals,
+    -mlongcalls,
+    -fstrict-volatile-bitfields,
+  ]
+  Compiler: /Users/YOU/.espressif/tools/xtensa-clang/esp-clang/bin/clang++
+```
+
+You should then get proper LSP integration via clangd.