Added README.md and command line interface.

README.md

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+# OMG
+_OMG - Obsidian Metadata Generator_
+
+OMG is a simple python script that generates metadata like the "Metadata Extractor"-plugin does.
+It cannot do as much as the plugin because it was made to generate metadata for
+[Perlite](https://github.com/secure-77/Perlite).
+
+## Setup
+Before you can use the script, you have to install the dependencies.
+Normally, you would install the dependencies like this:
+
+```shell
+pip install -r requirements.txt
+```
+
+(which also works on this project)
+
+But since OMG has only one dependency, (pyyaml) you can simply run
+
+```shell
+pip install pyyaml
+```
+
+## Usage
+
+### Command Line
+The script is usable from the command line. It takes 2 arguments: `vault_path` and `metadata_path`.
+`metadata_path` comes after `vault_path` and is optional because it has a default value of `vault_path/metadata.json`.
+
+### Python API
+To use the script as a module, simply import it and create an instance of the OMG class with the `path`-parameter set
+to your vault path. The metadata is stored as a dict in `instance.md_files`.
+
+Example:
+
+```python
+from omg import OMG
+
+metadata = OMG("/home/user/.obsidian/myVault")
+as_dict = metadata.md_files
+```
+
+To write the metadata, you can use `instance.dump(path: Pathlike=None, indent=2)`.
+This will create a `metadata.json` in your vault by default, but you can also specify any other path.
+
+Example:
+
+```python
+from omg import OMG
+
+metadata = OMG("/home/user/.obsidian/myVault")
+metadata.dump()
+```