Add python dependency install instructions

BUILDING.md

@@ -22,6 +22,18 @@ Install the RV32 target:
 rustup target add riscv32i-unknown-none-elf
 ```
 
+### Python
+
+Get a recent version of python3, make sure pip is installed. Maybe make a venv
+for this? amaranth isn't really that bad for polluting namespaces so don't worry
+about making a virtual environment unless you have firm opinions.
+
+Then:
+
+```shell
+pip install -r requirements.txt
+```
+
 ### ecpdap (Optional?)
 
 You may or may not have to install your own version of ecpdap, I found the one

requirements.txt

@@ -1,3 +1,4 @@
 git+https://github.com/amaranth-lang/amaranth
 git+https://github.com/amaranth-lang/amaranth-soc
 git+https://github.com/minerva-cpu/minerva
+git+https://github.com/amaranth-community-unofficial/amlib