Create README.md

Author: unterumarmung

README.md

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+# fixed_string
+C++ library that provides a `basic_fixed_string` template that combines `std::array` fixed-size semantic and `std::string` semantic together
+
+## Features
+
+* C++17 or higher
+* Header-only
+* Dependency-free
+* No dynamic allocations
+* Fully constexpr
+* Can be used as class non-type template parameter __(since C++20)__
+
+## Examples
+
+* Construction
+```cpp
+constexpr fixstr::fixed_string foo = "foo";
+```
+
+* Concatenation
+```cpp
+using namespace fixstr;
+constexpr fixed_string first = "Hello, ";
+constexpr fixed_string second = "World!";
+constexpr auto result = first + second; // "Hello, World!"
+```
+
+* Comparison
+```cpp
+using namespace fixstr;
+constexpr fixed_string first = "Hello, ";
+constexpr fixed_string second = "World!";
+static_assert(first == second); // false
+static_assert(first != second); // true
+static_assert(first < second);  // true
+static_assert(first <= second); // false
+static_assert(first > second);  // false 
+static_assert(first >= second); // false
+static_assert(first <=> second != 0); // true
+```
+
+* Non-type template parameter
+```cpp
+template <fixstr::fixed_string Foo>
+void bar()
+{
+    static_assert(Foo == "foo"sv);
+}
+
+void foo()
+{
+    bar<"foo">();
+}
+```
+
+## Integration
+Since it's a header only library, you need just copy `fixed_string.hpp` to your project.
+
+## Compiler compatibility
+* GCC >= 7.3
+* Clang >= 5
+* ICC >= 19.0.1
+* MSVC >= 14.28 / Visual Studio 2019 (I don't have access to older VS versions right now, so it can work on older versions too)
+
+**Using `basic_fixed_string` as class non-type template parameter full available in GCC >= 10**