add go mutext

Development/Go/strings.md

@@ -1,6 +1,6 @@
 # [strings](https://golang.org/pkg/strings/)
 
-Replace list of strings by using Replacer:
+Replace list of strings by using `Replacer`:
 
 ```go
 template := "My city is {city} and my country is {country}"

Development/Go/sync.md

@@ -0,0 +1,43 @@
+# [sync](https://golang.org/pkg/sync/)
+
+Use `Mutext` to prevent accessing variable from different goroutine at the same time:
+
+```go
+type foo struct {
+    mu sync.Mutext
+
+    bar map[string]string
+}
+
+func (f *foo) doSomething {
+    f.mu.Lock()
+    defer f.mu.Unlock()
+
+    // do the thing
+}
+
+```
+
+Use `RWMutex` if there are many readers, and few writers:
+
+```go
+type foo struct {
+    mu sync.RWMutext
+
+    bar map[string]string
+}
+
+func (f *foo) read {
+    f.mu.RLock()
+    defer f.mu.RUnlock()
+
+    // read
+}
+
+func (f *foo) write {
+    f.mu.Lock()
+    defer f.mu.Unlock()
+
+    // write
+}
+```