Consolidate instructions

BUILD.md

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+# Build instructions
+
+Typically build the CBDT then the COLRv1 as COLRv1 copies some information from CBDT.
+
+## CBDT
+
+```bash
+rm -rf venv  # in case you have an old borked venv!
+python3 -m venv venv
+source venv/bin/activate
+pip install -r requirements.txt
+rm -rf build/ && time make -j 48
+# Should take 2-3 minutes to create noto-emoji/NotoColorEmoji.ttf
+
+mv *.ttf fonts/
+```
+
+## COLRv1
+
+```bash
+# If you are updating to a new Unicode rev, update configs
+(cd colrv1 && python colrv1_generate_configs.py)
+git diff colrv1/*.toml
+
+# Compile the fonts
+(cd colrv1 && rm -rf build/ && time nanoemoji *.toml)
+cp colrv1/build/NotoColorEmoji.ttf fonts/Noto-COLRv1.ttf
+cp colrv1/build/NotoColorEmoji-noflags.ttf fonts/Noto-COLRv1-noflags.ttf
+
+# Post-process them
+python colrv1_postproc.py
+```
+
+## Emojicompat
+
+TODO detailed instructions