Create README.md

Some development instructions (since even I forgot how to build Pester from scratch).

README.md

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+Pester
+======
+
+Simple, disposable alarms and timers for OS X.
+
+Building Pester
+---------------
+
+1. Clone this repository.
+2. ```cd /path/to/Pester```
+3. ```git submodule update --init --force```
+4. Copy the Growl 1.2.3 SDK framework to /Library/Frameworks (inside the 2.0.1 SDK ZIP file in `Framework/Legacy`)
+5. Open the project file (in the Source folder) in Xcode and build. 
+
+The corresponding Xcode version is mentioned in the version history, though there should be a reasonable amount of slop.  The main thing to break is Perl dependencies, as OS X tends to include at most 2 versions.  This is one reason why recent versions of Pester no longer support as many OS X versions as they once did (Apple’s free yearly updates, more aggressive deprecation policy, and my vanishing free time don’t help).
+
+Running tests
+-------------
+
+Pester’s test coverage is dismal, but some tests do exist for trickier bits such as the exception-y deserialization process and interaction of `Date::Manip` with OS X date formats.
+
+1. Select Product → Test in Xcode.
+
+Need ideas?
+-----------
+
+Open `Pester to do.oo3` in OmniOutliner — it contains many ideas I haven’t had time to implement.