Add a CONTRIBUTING.md file

CONTRIBUTING.md

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+# Contributing to ACMEd
+
+
+## Testing and bug report
+
+The first way to help is to actually use the software and [report any bug you encounter](https://github.com/breard-r/acmed/issues). Do not hesitate to test the limits.
+
+
+## Fix issues in dependencies
+
+### nix
+
+The [nix](https://crates.io/crates/nix) crate does not currently allow to retrieve an UID or GID from a user or group name, which prevents ACMEd to do so. A pull request has been made to `nix` in early 2018 but has not been merged yet.
+
+- https://github.com/nix-rust/nix/pull/864
+
+### rust-openssl
+
+The [openssl](https://crates.io/crates/openssl) does not expose the Asn1Time in a usable way, which requires ACMEd to hack around it by parsing the string representation of such objects. This is sub-optimal and has already caused at least one bug.
+
+- https://github.com/sfackler/rust-openssl/issues/687
+- https://github.com/sfackler/rust-openssl/pull/673
+
+
+## Improving the code
+
+As a one-man project, it has several goals already set but not explicitly written in an issue or any other follow-up file. It will not be the case before version 1.0 is released since everything may change at any moment. Therefore, it is recommended to request change instead of implementing them, this way we can discuss how things should be made.
+
+If you really want to submit a pull request, please :
+
+- document your changes in the man pages and the `CHANGELOG.md` file
+- write as much tests as you can
+- run `cargo test` and be sure every test pass
+- format your code using [rustfmt](https://github.com/rust-lang/rustfmt)
+- be sure not to have any warning when compiling
+- run [clippy](https://github.com/rust-lang/rust-clippy) and fix any issue
+- refrain from including a new dependency (crates having `ring` in their dependency tree are an absolute no-go)
+- beware of potential repercussions on the default hooks: those should remain usable
+
+Not following the rules above will delay the merge since they will have to be fixed first.
+
+
+## Author vs. contributor
+
+Some people have troubles seeing the difference between an author and a contributor. Here is how it is seen withing this project.
+
+A contributor is a person who helps the project in various ways whenever she or he wants. As such, a contributor does not have any obligation other than being respectful and open-minded. People who wrote code that have been accepted are automatically listed in the [contributors page](https://github.com/breard-r/acmed/graphs/contributors). The creation of a file with the names of people contributing outside of the code base will be studied upon request from such people.
+
+An author is a person who has some responsibilities on the project. Authors are expected to contribute on a regular basis, decide architectural choices, enforce copyright issues and so on. One does not grant himself the author status, it is given by the others authors after having discussed the request.