README: update help text, repo link
README: update help text, repo link

Change-Id: Iebcd05932572908abe4fcd0bb39a04dde9325199

ttyangf/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp,ttyangf/libwebp,ttyangf/libwebp,PKRoma/libwebp,PKRoma/libwebp,ttyangf/libwebp,PKRoma/libwebp,ttyangf/libwebp