Add documentation for ifcpatch.

src/ifcpatch/README.md

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+# ifcpatch
+
+`ifcpatch` is a little CLI utility and library that lets you run a predetermined
+modification on an IFC file, known as a patch recipe. This is great for
+distributing little scripts that need to modify an IFC to users who don't know
+how to code or aren't interested in knowing the details.
+
+## Installation
+
+`ifcpatch` is a simple Python module. No compilation or packaging is necessary.
+Just download the `ifcpatch` directory and add it to your Python site packages.
+Then, you can run it as a CLI like:
+
+```
+$ python -m ifcpatch
+```
+
+If you want something more Unix-like ...
+
+```
+$ alias ifcpatch='python -m ifcpatch'
+```
+
+## Usage
+
+Usage is like any other CLI app.
+
+```
+$ ifcpatch -h
+
+usage: __main__.py [-h] -i INPUT [-o OUTPUT] -r RECIPE [-l LOG]
+                   [-a ARGUMENTS [ARGUMENTS ...]]
+
+Patches IFC files to fix badly formatted data
+
+optional arguments:
+  -h, --help            show this help message and exit
+  -i INPUT, --input INPUT
+                        The IFC file to patch
+  -o OUTPUT, --output OUTPUT
+                        The output file to save the patched IFC
+  -r RECIPE, --recipe RECIPE
+                        Name of the recipe to use when patching
+  -l LOG, --log LOG     Specify a log file
+  -a ARGUMENTS [ARGUMENTS ...], --arguments ARGUMENTS [ARGUMENTS ...]
+                        Specify custom arguments to the patch recipe
+```
+
+Exactly how it is run depends on the recipe. A recipe may require zero or more
+arguments which are specific to the recipe. Here's an example which runs the
+`ExtractElements` recipe, which, as the same suggests, extracts out elements.
+This recipe expects one argument, which is a [filtering
+query](https://wiki.osarch.org/index.php?title=IfcOpenShell_code_examples#IFC_Query_Syntax).
+In this example, we'll extract out all `IfcWall` elements.
+
+```
+$ ifcpatch -i input.ifc -o output.ifc -r ExtractElements -a ".IfcWall"
+```
+
+You can also use it as a library.
+
+```
+import ifcpatch
+ifcpatch.execute({
+    "input": "input.ifc",
+    "output": "output.ifc",
+    "recipe": "ExtractElements",
+    "arguments": [".IfcWall"],
+})
+```