Usage guide.

README.md

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+## Usage
+
+```julia
+using ReadWriteLocks
+
+# The type provided by this package is ReadWriteLock.
+# It has a single constructor.
+rwlock = ReadWriteLock()
+
+# This lock provides access to a read lock and a write lock
+rlock = read_lock(rwlock)
+wlock = write_lock(rwlock)
+
+# To acquire a read lock:
+lock!(rlock)
+
+# To release a read lock:
+unlock!(rlock)
+
+#=
+Write locks provide the same interface.
+
+You can acquire any number of read locks at a time.
+A read lock will block if a write lock has been acquired. 
+A write lock will block if a read lock or write lock has been acquired.
+=#
+```
+
 ## Compatibility
 
 This package is meant to be compatible with Julia's lightweight threads (where it is not strictly necessary) and true multithreaded Julia, in order to facilitate unified codebases that support future thread-safety.