Add Pull Request Template (#4247)

.github/PULL_REQUEST_TEMPLATE.md

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+# Your PR Title (What it does)
+
+<!--
+Thank you for contributing to LangChain! Your PR will appear in our next release under the title you set. Please make sure it highlights your valuable contribution.
+
+Replace this with a description of the change, the issue it fixes (if applicable), and relevant context. List any dependencies required for this change.
+
+After you're done, someone will review your PR. They may suggest improvements. If no one reviews your PR within a few days, feel free to @-mention the same people again, as notifications can get lost.
+-->
+
+<!-- Remove if not applicable -->
+
+Fixes # (issue)
+
+## Before submitting
+
+<!-- If you're adding a new integration, include an integration test and an example notebook showing its use! -->
+
+## Who can review?
+
+Community members can review the PR once tests pass. Tag maintainers/contributors who might be interested:
+
+<!-- For a quicker response, figure out the right person to tag with @
+
+        @hwchase17 - project lead
+
+        Tracing / Callbacks
+        - @agola11
+
+        Async
+        - @agola11
+
+        DataLoader Abstractions
+        - @eyurtsev
+
+        LLM/Chat Wrappers
+        - @hwchase17
+        - @agola11
+
+        Tools / Toolkits
+        - @vowelparrot
+ -->