Update README.md

add answer link

Author: Animadversio

README.md

@@ -8,4 +8,4 @@
 ## Colab notebooks
 * Playing with Stable Diffusion and inspecting the internal architecture of the models. [Open in Colab](https://colab.research.google.com/drive/1TvOlX2_l4pCBOKjDI672JcMm4q68sKrA?usp=sharing)
 * Build your own Stable Diffusion UNet model from scratch in a notebook. (with < 300 lines of codes!) [Open in Colab](https://colab.research.google.com/drive/1mm67_irYu3qU3hnfzqK5yQC38Fd5UFam?usp=sharing)
-* Build a Diffusion model (with UNet + cross attention) and train it to generate MNIST images based on the "text prompt". [Open in Colab](https://colab.research.google.com/drive/1Y5wr91g5jmpCDiX-RLfWL1eSBWoSuLqO?usp=sharing)
+* Build a Diffusion model (with UNet + cross attention) and train it to generate MNIST images based on the "text prompt". [Open in Colab (exercise)](https://colab.research.google.com/drive/1Y5wr91g5jmpCDiX-RLfWL1eSBWoSuLqO?usp=sharing) [Open in Colab (answer)](https://colab.research.google.com/drive/1_MEFfBdOI06GAuANrs1b8L-BBLn3x-ZJ?usp=sharing)