Add initial README with flatbuffers explanations

Author: Tristramg

README.md

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+# Library for Linear Reference System
+
+TODO: explain what this library is for
+
+## Using liblrs
+
+### Data serialization with FlatBuffers
+
+The data that defines an LRS is serialized using the [FlatBuffers format](https://flatbuffers.dev/).
+
+The schema is described in [schema/lrs.fbs](schema/lrs.fbs). The library is written in rust and the [generated file](src/lrs_generated.rs) is commited. This means there is no need to have the `flatc` executable to build and run this project.
+
+If your contribution changes the schema, you will need to generate the file with
+
+`flatc -o src --rust schema/lrs.fbs`