Remove a newline from README.txt so that when llvm.org gets merged in in the future, we will get a merge commit.
This is necessary since although currently llvm.org/compiler-rt and
github/swift-compiler-rt are the same, they may not be in the future.
In such a case, we want to cut compiler-rt stable branches from github, not from
llvm.org. To be able to do that in an automated fashion, the swift merge tool
needs to be able to reason about whether or not our internal branch had a commit
coming from llvm.org/compiler-rt or github/swift-compiler-rt. The easiest way to
do that is by ensuring that merging llvm.org/compiler-rt into
github/swift-compiler-rt does not fast forward.