Document git-lfs dependency in readme and/or check for git-lfs in `gradlew`

You must have git-lfs installed *before cloning the repo* in order to build from source. Otherwise only the pointer file is cloned.

For example, without git-lfs the gradle/wrapper/gradle-wrapper.jar local file contents are below, and `./gradlew build` fails with an uninformative error.

```
# cat gradle/wrapper/gradle-wrapper.jar
version https://git-lfs.github.com/spec/v1
oid sha256:e996d452d2645e70c01c11143ca2d3742734a28da2bf61f25c82bdc288c9e637
size 59203

# ./gradlew build
Error: Could not find or load main class org.gradle.wrapper.GradleWrapperMain
Caused by: java.lang.ClassNotFoundException: org.gradle.wrapper.GradleWrapperMain
```

I'm on Ubuntu and can install git-lfs with `sudo apt install git-lfs`. I can check that it's installed with `which git-lfs`. I'm not sure how to make that portable across platforms though.