Add contributing guidelines

CONTRIBUTING.md

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+# Contributing Guidelines
+
+## Legal
+
+Contributions must be all your own work; plagiarism is not allowed.
+By submitting a pull request, you agree to license your contribution
+under the [license of this repository](https://github.com/TheAlgorithms/Algorithms-Explanation/blob/master/LICENSE.md).
+
+## Translations
+
+Translations go in the appropriate folder named by the locale code.
+
+Do not change the structure or formatting of the original (English) explanation when translating or updating translations.
+You may change number formatting to fit the locale
+(`42.33` may be formatted as `42,33` in a german translation for instance)
+as long as it doesn't create ambiguities
+(e.g. `[42,33, 13]` in an array would be disallowed and should either remain `[42.33, 13]` or use a different delimiter `[42,33; 13]`).
+
+## Writing Explanations
+
+See the [explanation of the Euclidean Algorithm](en/Basic%20Math/Euclidean%20algorithm.md)
+for an example of how a good explanation may look like.
+
+### Structure
+
+You should structure your explanations using headings.
+The top-level heading should be the name of the algorithm or data structure to be explained
+
+Subsequent sub-headings *may* be:
+
+1. Problem solved by the algorithm
+2. Design/approach of the algorithm
+3. Detailed (yet not too technical) description of the algorithm, usually including (pseudo)-code
+4. Analysis (proof of correctness, best/worst/average cases, time & space complexity)
+5. Walkthrough(s) of how the algorithm processes example input(s)
+6. Application(s) of the algorithm
+7. Further resources such as reference implementations, videos or other explanations
+
+### Capitalization
+
+- Use *Title Case* for headings.
+- Start new sentences with a capital letter.
+
+### Typographical Conventions
+
+- Leave a space after punctuation such as `.`, `!`, `?`, `,`, `;` or `:`.
+- Add a space before and after `-`. **Do not add a space before punctuation.**
+- Add a space before an opening parenthesis `(`. Do not add a space before the closing parenthesis `)`.
+- Add spaces around (but not inside) quotes. Use single quotes for quotes within quotes.
+
+### Markdown Conventions
+
+[GitHub-flavored Markdown is used](https://github.github.com/gfm/). Explanations should render well when viewed from GitHub.
+
+- **Do not add redundant formatting.** Formatting should always be meaningful.
+  If you apply a certain formatting to all elements of a certain kind (e.g. adding emphasis around all headings), you're doing something wrong.
+- Use ATX-style "hashtag" headings: `#`, `##`, `###`, `####`, `#####`, `######` rather than Setext-style "underline" headings.
+  Leave blank lines around headings. The first heading should always be `# Title`. Subheadings must be exactly one level deeper than their parents.
+- Indent lists by two blanks. Prefer `-` over `*` for bulleted lists. Enumerate numbered lists correctly, starting at `1`.
+- Use fenced code blocks (and specify the correct language) rather than using indented code blocks.
+  Format code inside fenced code blocks properly (prefer pseudocode over code though). Leave blank lines around fenced code blocks.
+- Use named links `[name](link)` rather than relying on automatic link detection or using `<>`-links.
+  There are rarely good reasons to not give a link a descriptive name.
+- Use HTML only if necessary (rarely - if ever - the case). Do not use HTML for unnecessary formatting.