add contribution guidelines and github default config (#16)

motivation: adopt contribution guidelines and github PR conventions

changes:
* add contributing guidelines, including commit message template
* add github PR guidelines, including PR message template

.github/ISSUE_TEMPLATE.md

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+### Expected behavior
+_[what you expected to happen]_
+
+### Actual behavior
+_[what actually happened]_
+
+### Steps to reproduce
+
+1. ...
+2. ...
+
+### If possible, minimal yet complete reproducer code (or URL to code)
+
+_[anything to help us reproducing the issue]_
+
+### SwiftMetrics version/commit hash
+
+_[the SwiftMetrics tag/commit hash]_
+
+### Swift & OS version (output of `swift --version && uname -a`)

.github/PULL_REQUEST_TEMPLATE.md

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+_[One line description of your change]_
+
+### Motivation:
+
+_[Explain here the context, and why you're making that change. What is the problem you're trying to solve.]_
+
+### Modifications:
+
+_[Describe the modifications you've done.]_
+
+### Result:
+
+_[After your change, what will change.]_

CONTRIBUTING.md

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+## Legal
+
+By submitting a pull request, you represent that you have the right to license
+your contribution to Apple and the community, and agree by submitting the patch
+that your contributions are licensed under the Apache 2.0 license (see
+`LICENSE.txt`).
+
+## How to submit a bug report
+
+Please ensure to specify the following:
+
+* SwiftMetrics commit hash
+* Contextual information (e.g. what you were trying to achieve with SwiftMetrics)
+* Simplest possible steps to reproduce
+  * More complex the steps are, lower the priority will be.
+  * A pull request with failing test case is preferred, but it's just fine to paste the test case into the issue description.
+* Anything that might be relevant in your opinion, such as:
+  * Swift version or the output of `swift --version`
+  * OS version and the output of `uname -a`
+  * Network configuration
+
+### Example
+
+```
+SwiftMetrics commit hash: b17a8a9f0f814c01a56977680cb68d8a779c951f
+
+Context:
+While testing my application that uses with SwiftMetrics, I noticed that ...
+
+Steps to reproduce:
+1. ...
+2. ...
+3. ...
+4. ...
+
+$ swift --version
+Swift version 4.0.2 (swift-4.0.2-RELEASE)
+Target: x86_64-unknown-linux-gnu
+
+Operating system: Ubuntu Linux 16.04 64-bit
+
+$ uname -a
+Linux beefy.machine 4.4.0-101-generic #124-Ubuntu SMP Fri Nov 10 18:29:59 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux
+
+My system has IPv6 disabled.
+```
+
+## Writing a Patch
+
+A good SwiftMetrics patch is:
+
+1. Concise, and contains as few changes as needed to achieve the end result.
+2. Tested, ensuring that any tests provided failed before the patch and pass after it.
+3. Documented, adding API documentation as needed to cover new functions and properties.
+4. Accompanied by a great commit message, using our commit message template.
+
+### Commit Message Template
+
+We require that your commit messages match our template. The easiest way to do that is to get git to help you by explicitly using the template. To do that, `cd` to the root of our repository and run:
+
+    git config commit.template dev/git.commit.template
+
+### Make sure Tests work on Linux
+
+SwiftMetrics uses XCTest to run tests on both macOS and Linux. While the macOS version of XCTest is able to use the Objective-C runtime to discover tests at execution time, the Linux version is not.
+For this reason, whenever you add new tests **you have to run a script** that generates the hooks needed to run those tests on Linux, or our CI will complain that the tests are not all present on Linux. To do this, merely execute `ruby ./scripts/generate_linux_tests.rb` at the root of the package and check the changes it made.
+
+## How to contribute your work
+
+Please open a pull request at https://github.com/apple/swift-metrics. Make sure the CI passes, and then wait for code review.