add some documentation

Author: striezel

CONTRIBUTING.md

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+# Contribution guidelines
+
+First off, thanks for taking the time to contribute!
+
+The following is a set of guidelines for contributing to this project.
+These are not set in stone, but following these guidelines closely is usually
+considered good practice. Use your best judgment, and feel free to propose
+changes to this document in a pull request.
+
+## Using GitHub to implement features and fix bugs
+
+If you want to help out with one of our bugs or implement a new feature, here
+are some suggestions for success. The basic process looks like this:
+
+1. Fork the repository.
+2. Clone your fork to your development machine.
+3. Create your issue branch.
+4. Implement your feature or fix the bug and push the results to your fork.
+5. Send us a pull request.
+
+You can find more info about [creating a fork](https://help.github.com/articles/fork-a-repo/)
+and [pull request](https://help.github.com/articles/creating-a-pull-request-from-a-fork/)
+in the GitHub help articles. Our maintainers will review your pull request as
+soon as we can.
+
+## What makes a good pull request?
+
+To summarize:
+
+* Address only one issue with a single pull request.
+* For new functionality, have a conversation with the maintainers before putting
+  in too much work. Opening an issue is a good way to start the conversation.
+* Finally, small pull requests are preferred. That way we can find time to
+  review them between work, and family, and watching videos of cute cats jumping
+  into cardboard boxes on the internet.
+
+## Styleguides
+
+### Git Commit Messages
+
+* Use the present tense ("Add feature" not "Added feature").
+* Use the imperative mood ("Move cursor to..." not "Moves cursor to...").
+* Limit all lines of the commit message to 65 characters or less.
+  * Exception: When a line contains a URLs that is too long to fit into the
+    character limit, do not add a line break after 65 characters.
+* URLs (e. g. for linking to information about a vulnerability in a dependency)
+  should be enclose in angle brackets, e. g. `<https://example.com/>`.
+* For multi-line commit messages:
+  * Write the summary on the first line.
+  * Leave the second line empty.
+  * Then follow up with a more detailed explanation of the changes.
+* When only changing documentation, include `[ci skip]` on a separate line in
+  the commit message.
+
+### Code Styleguide
+
+* Try to limit lines to 80 characters.
+* Use spaces (not tabs) for indentation, two spaces per indentation level.
+* Try to follow best practices and guidelines for modern C++ (i. e. C++11 and
+  later). If in doubt, consult the C++ Core Guidelines at
+  <https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines>.

readme.md

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+# webp-viewer - a very simple WebP image viewer
+
+`webp-viewer` is a program that loads and displays a WebP image.
+
+Animations are currently not supported, and probably never will be.
+
+## Building from source
+
+### Prerequisites
+
+To build webp-viewer from source you need a C++ compiler with support for C++17,
+CMake 3.8 or later, OpenGL, the GLFW library and the libwebp library.
+pkg-config is required to make it easier to find compiler options for the
+installed libraries.
+
+It also helps to have Git, a distributed version control system, on your build
+system to get the latest source code directly from the Git repository.
+
+All of that can usually be installed be typing
+
+    apt-get install cmake g++ git libgl-dev libglfw3-dev libwebp-dev pkg-config
+
+or
+
+    yum install cmake gcc-c++ git glfw-devel libwebp-devel pkgconfig
+
+into a root terminal.
+
+### Getting the source code
+
+Get the source directly from Git by cloning the Git repository and change to
+the directory after the repository is completely cloned:
+
+    git clone https://gitlab.com/striezel/webp-viewer.git
+    cd webp-viewer
+
+That's it, you should now have the current source code on your machine.
+
+### Build process
+
+The build process is relatively easy, because CMake does all the preparations.
+Starting in the root directory of the source, you can do the following steps:
+
+    mkdir build
+    cd build
+    cmake ../
+    make -j4
+
+## Usage
+
+```
+webp-viewer [OPTIONS] [FILE]
+
+Loads and shows a WebP image.
+
+options:
+  -? | --help     - Shows this help message.
+  -v | --version  - Shows version information.
+  FILE            - Sets the file name of image to show.
+```
+
+## Copyright and Licensing
+
+Copyright 2022  Dirk Stolle
+
+This program is free software: you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation, either version 3 of the License, or
+(at your option) any later version.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with this program.  If not, see <http://www.gnu.org/licenses/>.