Add doc for folly/MapUtil

Summary: Add documentation for folly/MapUtil.h. Documentation provides a section on the motivation of the utility as well as some examples (mostly pulled from the unit tests)

Reviewed By: Gownta

Differential Revision: D62447206

fbshipit-source-id: fd869aa558ef38398248a15b7c196673d6cb9269

folly/docs/MapUtils.md

@@ -0,0 +1,80 @@
+`folly/container/MapUtil.h`
+---------------------------
+
+MapUtil provides convenience functions to get a value from a map.
+
+### Motivation & Comparisons
+
+Fetching a value in a map can be tedious. For example, `map.at()` will throw an exception if the key is not present:
+```cpp
+try {
+    auto& v = m.at(1);
+} catch (...) {
+    // do something!
+}
+```
+
+We can avoid the exception handling by using `find`:
+```cpp
+auto it = m.find(1);
+if (it != m.end()) {
+    auto& v = it->second;
+}
+```
+
+Both of these cases can be written more succinctly and without exception handling using folly/MapUtil.h
+```cpp
+if (const auto* v = folly::get_ptr(m, 1)) {
+    // do stuff with `v` which is guaranteed to be present in this block
+}
+```
+
+We can also easily lookup values in nested maps:
+```cpp
+std::map<int, std::map<int, int>> m {{1, {{2, 3}}}};
+// using find
+auto it1 = m.find(1);
+if (it1 != m.end()) {
+    auto it2 = it1->second.find(2);
+    if (it2 != it1->second.end()) {
+        auto& v = it2->second;
+    }
+}
+// using folly/MapUtil.h
+if (const auto* v = folly::get_ptr(m, 1, 2)) {
+    // do stuff with `v`!
+}
+```
+
+### Sample
+The corresponding unit test suite has a very thorough collection of examples: `folly/container/test/MapUtilTest.cpp`,
+for brevity, here is a small glimpse:
+
+***
+```cpp
+std::map<int, int> m {{1, 2}};
+
+// Return a pointer (nullptr if the key is not in the map)
+EXPECT_EQ(2, *get_ptr(m, 1));
+EXPECT_TRUE(get_ptr(m, 2) == nullptr);
+
+// Return a value (or default)
+EXPECT_EQ(2, get_default(m, 1, 42));
+EXPECT_EQ(42, get_default(m, 2, 42));
+
+// Return an optional (empty if the key is not in the map)
+EXPECT_TRUE(get_optional(m, 1).has_value());
+EXPECT_EQ(2, get_optional(m, 1).value());
+
+// Return a reference
+const int i = 42;
+EXPECT_EQ(2, get_ref_default(m, 1, i));
+EXPECT_EQ(42, get_ref_default(m, 2, i));
+
+// Even works for nested maps!
+std::map<int, map<int, map<int, map<int, int>>>> nested{{1, {{2, {{3, {{4, 5}}}}}}}};
+EXPECT_EQ(5, *get_ptr(nested, 1, 2, 3, 4));
+EXPECT_TRUE(get_ptr(nested, 1, 2, 3, 4));
+EXPECT_FALSE(get_ptr(nested, 1, 2, 3, 0));
+```
+***