README.md: document msync, mlock and munlock

files exposed under `/dev/shm`.  There doesn't seem to be a portable

method of doing that.

### `SharedArray.msync(array, flags)`

This function is a wrapper around `msync(2)` and is only useful when

using file-backed arrays (i.e. not POSIX shared memory). msync(2)

flushes the mapped memory region back to the filesystem. The `flags`

are exported as constants in the module definition (see below) and are

a 1:1 map of the `msync(2)` flags, please refer to the manual page of

`msync(2)` for details.

### `SharedArray.mlock(array)`

This function is a wrapper around `mlock(2)`: lock the memory map into

RAM, preventing that memory from being paged to the swap area.

### `SharedArray.munlock(array)`

This function is a wrapper around `munlock(2)`: unlock the memory map,

allowing that memory to be paged to the swap area.

## Constants

### `SharedArray.MS_ASYNC`

Flag for `SharedArray.msync()`. Specifies that an update be scheduled,

but the call returns immediately.

### `SharedArray.MS_SYNC`

Flag for `SharedArray.msync()`. Requests an update and waits for it to

complete.

### `SharedArray.MS_INVALIDATE`

Flag for `SharedArray.msync()`. Asks to invalidate other mappings of

the same file (so that they can be updated with the fresh values just

written).

## Requirements

* Python 2.7 or 3+