Adding missing `pipeline_tag` annotation to enable inference providers to register

pipeline_tag is transformer lingo for "how does this model operate at inference time?"

Setting this tag will allow inference providers (like featherless ai) to register to provide inference for this model.

README.md

@@ -1,6 +1,7 @@
 ---
 license: other
 license_link: https://huggingface.co/stepfun-ai/Step-5-Preview/blob/main/LICENSE.txt
+pipeline_tag: image-text-to-text
 ---
 
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