[doc] Model upload and sharing

ping @lysandrejik @thomwolf

Is this clear enough? Anything we should add?

docs/source/index.rst

@@ -58,6 +58,7 @@ The library currently contains PyTorch and Tensorflow implementations, pre-train
     installation
     quickstart
     pretrained_models
+    model_sharing
     examples
     notebooks
     serialization

docs/source/model_sharing.md

@@ -0,0 +1,40 @@
+# Model upload and sharing
+
+Starting with `v2.2.2`, you can now upload and share your fine-tuned models with the community, using the <abbr title="Command-line interface">CLI</abbr> that's built-in to the library.
+
+**First, create an account on [https://huggingface.co/join](https://huggingface.co/join)**. Then:
+
+```shell
+transformers-cli login
+# log in using the same credentials as on huggingface.co
+```
+Upload your model:
+```shell
+transformers-cli upload ./path/to/pretrained_model/
+
+# ^^ Upload folder containing weights/tokenizer/config
+# saved via `.save_pretrained()`
+
+transformers-cli upload ./config.json [--filename foobar.json]
+
+# ^^ Upload a single file
+# (you can optionally override its filename)
+```
+
+Your model will then be accessible through its identifier:
+```python
+"username/model_name"
+```
+
+Anyone can load it from code:
+```python
+tokenizer = AutoTokenizer.from_pretrained("username/model_name")
+model = AutoModel.from_pretrained("username/model_name")
+```
+
+Finally, list all your files on S3:
+```shell
+transformers-cli ls
+# List all your S3 objects.
+```
+