Create a maintenance mutex skeleton within luci-go

mmutex/README.md

Index: mmutex/README.md
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+Maintenance Mutex (mmutex)
+==========================
+
+mmutex is a command line tool that helps prevent maintenance tasks from running
+during user tasks. The tool does this by way of a global lock file that users 
+must acquire before running their tasks.
+
+Clients can use this tool to request that their task be run with one of two 
+types of access to the system:
+
+  * **Exclusive access** guarantees that no other callers have any access
+    exclusive or shared) to the resource while the specified command is run.
+  * **Shared access** guarantees that only other callers with shared access
+    will have access to the resource while the specified command is run.
+
+In short, exclusive access guarantees a task is run alone, while shared access
+tasks may be run alongside other shared access tasks.
+
+TODO(charliea): Document actual command line usage here.