docs: update Together README.md (#18004)

## PR message
**Description:** This PR adds a README file for the Together API in the
`libs/partners` folder of this repository. The README includes:
 - A brief description of the package
 - Installation instructions and class introductions
 - Simple usage examples

**Issue:** #17545 

This PR only contains document changes.

---------

Co-authored-by: Bagatur <baskaryan@gmail.com>

libs/partners/together/README.md

@@ -1 +1,48 @@
 # langchain-together
+
+This package contains the LangChain integration for Together's generative models.
+
+## Installation
+
+```sh
+pip install -U langchain-together
+```
+
+## Embeddings
+
+You can use Together's embedding models through `TogetherEmbeddings` class.
+
+```py
+from langchain_together import TogetherEmbeddings
+
+embeddings = TogetherEmbeddings(
+    model='togethercomputer/m2-bert-80M-8k-retrieval'
+)
+embeddings.embed_query("What is a large language model?")
+```
+
+## LLMs
+
+You can use Together's generative AI models as Langchain LLMs:
+
+```py
+from langchain_together import Together
+from langchain_core.prompts import PromptTemplate
+
+llm = Together(
+    model="togethercomputer/RedPajama-INCITE-7B-Base",
+    temperature=0.7,
+    max_tokens=64,
+    top_k=1,
+    # together_api_key="..."
+)
+
+template = """Question: {question}
+Answer: """
+prompt = PromptTemplate.from_template(template)
+
+chain = prompt | llm
+
+question = "Who was the president in the year Justin Beiber was born?"
+print(chain.invoke({"question": question}))
+```