Readme to markdown

[raoulcollenteur]

Short Description

Change the README.rst to README.md because GitHub is better at formatting this important page when written in markdown. I found out while adding a warning for 1.5. See result here: https://github.com/pastas/pastas/tree/readme_to_markdown

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
❌ -0.06% (target: +0.00%)	✅ ∅

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (c44c3e2)	6294	4780	75.95%
Head commit (f63fb3c)	6294 (+0)	4776 (-4)	75.88% (-0.06%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#743)	0	0	∅ (not applicable)

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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_{You may notice some variations in coverage metrics with the latest Coverage engine update. For more details, visit the documentation}