chore: add readme

README.md

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+# libvieter
+
+This library powers part of Vieter, most noteably the sections that can easily
+be implemented in C (or just parts I want to implement in C because it's fun).
+
+The goal of this library is to be completely self-contained, meaning any
+required data structures have to be implemented as well. It can only depend on
+the C standard libraries.
+
+Currently it contains the following:
+
+* Cron expression parser & next time calculator
+
+## Development
+
+### `compile_commands.json`
+
+Clangd requires a `compile_commands.json` to function properly. You can
+generate it using [bear](https://github.com/rizsotto/Bear):
+
+```sh
+make clean
+bear -- make
+```
+
+This will create a `compile_commands.json` file in the current directory.