adding readme to explain how to add new models

adding_new_model.md

+CoreML models can be registered and used automatically with the CoreMLEncoder. To add a new model:
+
+1. Add the .mlpackage file to ```Sources/SwiftNLP/Resources```.
+
+2. From the ```swiftnlp``` directory, run the following from the command line:
+    - ```xcrun coremlcompiler generate Sources/SwiftNLP/Resources/[name of mlpackage file]/ --language Swift Sources/SwiftNLPGenericLLMMacros```
+    
+    This should generate a .swift file.
+
+3. Add the following script steps to the ```.compile_models``` item in ```.gitlab-ci.yml``` file:
+    - ```xcrun coremlcompiler compile Sources/SwiftNLP/Resources/[name of mlpackage file]/ Sources/SwiftNLP/Models```
+    - ```xcrun coremlcompiler generate Sources/SwiftNLP/Resources/[name of mlpackage file]/ --language Swift Sources/SwiftNLP/Resources```
+    - ```mv Sources/SwiftNLP/Resources/[name of the generated swift file] Sources/SwiftNLP/2.\ Encoding```
+
+4. Navigate to ```Sources/SwiftNLPGenericLLMMacros/ModelClasses.swift```.
+    - Add the following to the `LLM_MODEL_CLASSES` map:
+        ```
+        "[string name of model]": [
+            LLMModelClassesKey.Input: [name of model input class].self,
+            LLMModelClassesKey.Output: [name of model output class].self,
+            LLMModelClassesKey.Model: [name of model class].self,
+            LLMModelClassesKey.FeatureName: [name of feature to use],
+            LLMModelClassesKey.URL: "[name of ml package file without .mlpackage].mlmodelc",
+            LLMModelClassesKey.InputDimension: [input size]
+        ]
+        ```
+    - Notes on how to perform the above step:
+        - [string name of model] can be any string you want.
+        - the [name of model input class], [name of model output class], and [name of model class] can all be retrieved from the generated swift file. They are the names of the classes present in that file.
+        - the name of the feature to use is present in the model output class. Typically, it's `embeddings` though some models may have other fields like `pooler_output`.
+        - the [input size] can be retrieved from the auto-generated documentation in the model input class.
+
+5. Build the project, and the model can now be used anywhere. To access the model, instantiata an `LLMEmbedings` object with the model's string identifier. If using the CoreMLEncoder, set its `model` field to the model's string identifier.
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