Add issue template and contribution rules

.github/ISSUE_TEMPLATE.md

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+<!--
+
+Thank you for submitting an issue!
+
+Please state the acme4j version you are using, so we know whether the bug is related to the ACMEv1 or ACMEv2 protocol.
+
+-->

CONTRIBUTING.md

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+# Contributing to _acme4j_
+
+Thank you for taking your time to contribute!
+
+## Acceptance Criteria
+
+These criteria must be met for a successful pull request:
+
+* Follow the [Style Guide](#Style-Guide).
+* If you add code, remember to add [unit tests](#Unit-Tests) that test your code.
+* All unit tests must run successfully.
+* Integration tests should run successfully, unless there is a good reason (e.g. waiting for a pending change in Pebble).
+* Your commits follow the [git commit](#git-Commits) guide.
+* You accept that your code is distributed under the terms of [Apache License 2.0](http://www.apache.org/licenses/LICENSE-2.0).
+
+## Style Guide
+
+Our style guide bases on [Oracle's Code conventions for the Java Programming Language](http://www.oracle.com/technetwork/java/codeconventions-150003.pdf). These additional rules apply:
+
+* Indentation is 4 spaces. Do not use tabs!
+* Remove trailing spaces.
+* Line length is 90 characters. You may exceed this length by a few characters if it is easier to read than a wrapped line.
+* `if`, `for` and `while` statements always use blocks, even for a single statement.
+* All types and methods must have a descriptive JavaDoc, except of `@Override` annotated methods. For plain getter and setter methods, `@param` and '@return' can be omitted.
+
+## Unit Tests
+
+More than 80% of the code is covered by unit tests, and we would like to keep it that way.
+
+* Main functionalities must be covered by unit tests.
+* Corner cases should be covered by unit tests.
+* Common exception handling does not need to be tested.
+* No tests are required for code that is not expected to be executed (e.g. `UnsupportedEncodingException` when handling utf-8, or the empty private default constructor of a utility class).
+* Unit tests should not depend on external resources, as they might be temporarily unavailable at runtime.
+
+There are no unit tests required for the `acme4j-example` and `acme4j-it` modules.
+
+## git Commits
+
+Good programming does not end with a clean source code, but should have pretty commits as well.
+
+* Always put separate concerns into separate commits.
+* If you have interim commits in your history, squash them with an interactive rebase before sending the pull request.
+* Use present tense and imperative mood in commit messages ("fix bug #1234", not "fixed bug #1234").
+* Always give meaningful commit messages (not just "bugfix").
+* The commit message must be concise and should not exceed 50 characters. Further explanations may follow in subsequent lines, with an empty line as separator.
+* Commits must compile and must not break unit tests.