Update compilation.md with automated script.

[ccorcos]

No description provided.

[mceachen]

Thanks for taking the time to write this PR!

Unfortunately, I suspect people will blindly copy and paste this in the future and not realize they're pulling in an out of date amalgamation (this library is popular, and people open spurious bugs continuously. I suspect this will be a new source.)

If you want to be fancy, you can do something like this, which would be evergreen.

1. do the npm install better-sqlite3 first.
2. then mkdir -p sqlite-amalgamation && cd sqlite-amalgamation
3. then something like this:

source <(egrep "^(YEAR|VERSION)=" ../node_modules/better-sqlite3/deps/download.sh)
curl https://sqlite.org/$YEAR/sqlite-amalgamation-$VERSION.tar.gz | tar -xz --strip 1

To fetch and extract into your new dir.

[ccorcos]

That works for me!