Update CONTRIBUTION guidelines and PR Template (#5140)

# Update contribution guidelines and PR template

This PR updates the contribution guidelines to include more information
on how to handle optional dependencies. 

The PR template is updated to include a link to the contribution guidelines document.

.github/CONTRIBUTING.md

@@ -115,8 +115,37 @@ To get a report of current coverage, run the following:
 make coverage
 ```
 
+### Working with Optional Dependencies
+
+Langchain relies heavily on optional dependencies to keep the Langchain package lightweight.
+
+If you're adding a new dependency to Langchain, assume that it will be an optional dependency, and
+that most users won't have it installed.
+
+Users that do not have the dependency installed should be able to **import** your code without
+any side effects (no warnings, no errors, no exceptions). 
+
+To introduce the dependency to the pyproject.toml file correctly, please do the following: 
+
+1. Add the dependency to the main group as an optional dependency
+  ```bash
+  poetry add --optional [package_name]
+  ```
+2. Open pyproject.toml and add the dependency to the `extended_testing` extra
+3. Relock the poetry file to update the extra.
+  ```bash
+  poetry lock --no-update
+  ```
+4. Add a unit test that the very least attempts to import the new code. Ideally the unit
+test makes use of lightweight fixtures to test the logic of the code.
+5. Please use the `@pytest.mark.requires(package_name)` decorator for any tests that require the dependency.
+
 ### Testing
 
+See section about optional dependencies.
+
+#### Unit Tests
+
 Unit tests cover modular logic that does not require calls to outside APIs.
 
 To run unit tests:
@@ -133,8 +162,20 @@ make docker_tests
 
 If you add new logic, please add a unit test.
 
+
+
+#### Integration Tests
+
 Integration tests cover logic that requires making calls to outside APIs (often integration with other services).
 
+**warning** Almost no tests should be integration tests. 
+
+  Tests that require making network connections make it difficult for other
+  developers to test the code.
+
+  Instead favor relying on `responses` library and/or mock.patch to mock
+  requests using small fixtures.
+
 To run integration tests:
 
 ```bash

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,7 +1,7 @@
 # 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.
+Thank you for contributing to LangChain! Your PR will appear in our 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.
 
@@ -14,7 +14,17 @@ Fixes # (issue)
 
 ## Before submitting
 
-<!-- If you're adding a new integration, include an integration test and an example notebook showing its use! -->
+<!-- If you're adding a new integration, please include:
+
+1. a test for the integration - favor unit tests that does not rely on network access.
+2. an example notebook showing its use
+
+
+See contribution guidelines for more information on how to write tests, lint
+etc:
+
+https://github.com/hwchase17/langchain/blob/master/.github/CONTRIBUTING.md
+-->
 
 ## Who can review?