Adding Gotchas in README.md (#3138)

Summary:
Pull Request resolved: #3138

Populating based on feedback from George from Arm

Created from CodeHub with https://fburl.com/edit-in-codehub

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Reviewed By: digantdesai

Differential Revision: D56319098

fbshipit-source-id: 6c15ef3c2cb3857b58c21d7b58a0cdf36077ee9d

Author: mergennachin

examples/models/llama2/README.md

@@ -263,12 +263,13 @@ This example tries to reuse the Python code, with minimal modifications to make
 3. No dependencies on fairscale. The ColumnParallelLinear, ParallelEmbedding and training are not needed and supported in ExecuTorch.
 
 
-# Clean
-To clean your build:
+# Common Issues and Mitigations:
+- To clean your build:
 ```
 git clean -xfd
 pip uninstall executorch
 ./install_requirements.sh <options>
 
 rm -rf cmake-out
 ```
+- If you encounter `pthread` related issues during link time, add `pthread` in `target_link_libraries` in `CMakeLists.txt`