Expand README with structured deep learning notes outline

[Copilot]

The repository had a near-empty README. Replaced it with a topic-organized reference covering core DL concepts.

Changes

• Fundamentals – linear algebra, probability, gradient descent, backprop
• Neural Networks – MLP, activation functions, optimizers (SGD, Adam, RMSProp)
• CNNs – convolution/pooling, LeNet → ResNet lineage, transfer learning
• RNNs – vanilla RNN, LSTM, GRU, seq2seq
• Transformers – self-attention, Transformer architecture, BERT/GPT overview
• Regularization – dropout, batch norm, weight decay, early stopping
• Tools – NumPy, PyTorch, TensorFlow/Keras
• References – Deep Learning book, fast.ai, Stanford CS231n

Original prompt

this is a learning deep learning notes

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