README: Add devDependencies badge

Change-Id: I7021e81907e59e2f2fcc0df83e0caa28b5cd33eb
Replace boolean checks of getSlugAtOffset with hasSlugAtOffset

Change-Id: I68e295599d4b66642c7d81447e0ba0ccbe1602c4
Merge "Replace boolean checks of getSlugAtOffset with hasSlugAtOffset"
Merge "README: Add devDependencies badge"