feat(week1): add docs and task for week 1.

csrc/hello_world.cu

@@ -0,0 +1,21 @@
+#include <cstdio>
+#include <cuda_runtime.h>
+
+__global__ void hello() {
+  int tid = blockIdx.x * blockDim.x + threadIdx.x;
+
+  for (int i = 0; i < 8; i++) {
+    if (tid == i) {
+      printf("Hello from thread %d\n", tid);
+    }
+    __syncthreads();
+  }
+}
+
+int main() {
+  int nthreads = 8;
+  int nblocks = 1;
+  hello<<<nblocks, nthreads>>>();
+  cudaDeviceSynchronize();
+  return 0;
+}