Docs: add memory manager example

Author: qorexdevs

docs/user_guide.md

@@ -1370,6 +1370,38 @@ a report on the number of allocations, bytes used, etc.
 This data will then be reported alongside other performance data, currently
 only when using JSON output.
 
+For example, a custom manager can report allocation data collected from
+allocator hooks or another profiler:
+
+```c++
+class MyMemoryManager : public benchmark::MemoryManager {
+ public:
+  void Start() override {}
+
+  void Stop(Result& result) override {
+    result.num_allocs = 42;
+    result.max_bytes_used = 42000;
+  }
+};
+
+static void BM_work(benchmark::State& state) {
+  for (auto _ : state) {
+    /* ... */
+  }
+}
+BENCHMARK(BM_work);
+
+int main(int argc, char** argv) {
+  benchmark::MaybeReenterWithoutASLR(argc, argv);
+  MyMemoryManager mm;
+  benchmark::RegisterMemoryManager(&mm);
+  benchmark::Initialize(&argc, argv);
+  benchmark::RunSpecifiedBenchmarks();
+  benchmark::RegisterMemoryManager(nullptr);
+  benchmark::Shutdown();
+}
+```
+
 <a name="profiling" />
 
 ## Profiling