Update README.md

docs: Add testing guidelines for contributors  
- Included structured approach based on Berard's methodology.  
- Detailed steps for writing test cases, including examples for:  
  - Purpose  
  - Specified states  
  - Messages/operations  
  - Exceptions  
  - External conditions  
  - Supplementary information  
- Ensured clarity and modularity for ease of understanding.

libs/text-splitters/README.md

@@ -36,3 +36,59 @@ Patch version increases will occur for:
 As an open-source project in a rapidly developing field, we are extremely open to contributions, whether it be in the form of a new feature, improved infrastructure, or better documentation.
 
 For detailed information on how to contribute, see the [Contributing Guide](https://python.langchain.com/docs/contributing/).
+
+
+## Writing Test Case Guidelines
+
+To ensure high-quality and maintainable testing, we follow a structured approach. Below is a step-by-step guideline for writing test cases effectively:
+
+---
+
+### Example: `test_character_text_splitter_keep_separator_regex`
+
+#### **1. Unique Identifier**
+- Use a descriptive method name in `snake_case` format.  
+  **Example:** `test_character_text_splitter_keep_separator_regex`.
+
+#### **2. Purpose**
+- Clearly state the functionality or edge case being tested. Include this as a docstring after the test method definition.  
+  **Example:**  
+  > *To test the behavior of `CharacterTextSplitter` when a separator is a regex special character and needs to be retained in the output chunks.*
+
+#### **3. Testing Steps**
+Each test should include the following details:
+
+- **a. Specified States:**  
+  Clearly define the inputs, initial configurations, and state of the system.  
+  **Example:**  
+  ```plaintext
+  Initial text: "foo.bar.baz.123"  
+  Separator: "." (escaped or plain based on `is_separator_regex`)  
+  Chunk size: 1  
+  Chunk overlap: 0  
+  Keep separator: True  
+
+- **b. Messages/Operations:**
+  List the specific operations or methods invoked in the test.
+  **Example:**
+  
+  1. Initialize `CharacterTextSplitter` with the given configurations.  
+  2. Call `splitter.split_text(text)`.
+ 
+
+- **c. Exceptions (Optional):**
+  Mention any expected exceptions, if applicable.
+
+- **d. External Conditions (Optional):**
+  Specify any external dependencies or conditions needed for the test.
+  **Example:** Requires Python's `re` module for `regex-based splitting`.
+
+- **e. Supplementary Information:**
+  State the **expected output** or any additional details to help understand the test.
+  **Example:**
+  Expected result: `["foo", ".bar", ".baz", ".123"]`
+
+  
+#### **4. Clarity and Modularity**
+  Each test case should focus on testing one specific behavior or edge case.
+  The test setup, operations, and assertions should be concise and easy to follow.