doc: initial RapidCheck property-based testing documentation

Author: jonatack

doc/rapidcheck.md

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+# RapidCheck property-based testing for Bitcoin Core
+
+## Concept
+
+Property-based testing is experimentally being added to Bitcoin Core with
+[RapidCheck](https://github.com/emil-e/rapidcheck), a C++ framework for
+property-based testing inspired by the Haskell library
+[QuickCheck](https://hackage.haskell.org/package/QuickCheck).
+
+RapidCheck performs random testing of program properties. A specification of the
+program is given in the form of properties which functions should satisfy, and
+RapidCheck tests that the properties hold in a large number of randomly
+generated cases.
+
+If an exception is found, RapidCheck tries to find the smallest case, for some
+definition of smallest, for which the property is still false and displays it as
+a counter-example. For example, if the input is an integer, RapidCheck tries to
+find the smallest integer for which the property is false.
+
+## Running
+
+If RapidCheck is installed, Bitcoin Core will automatically run the
+property-based tests with the unit tests during `make check`, unless the
+`--without-rapidcheck` flag is passed when configuring.
+
+For more information, run `./configure --help` and see `--with-rapidcheck` under
+Optional Packages.
+
+## Setup
+
+The following instructions have been tested with Linux Debian and macOS.
+
+1. Clone the RapidCheck source code and cd into the repository.
+
+    ```shell
+    git clone https://github.com/emil-e/rapidcheck.git
+    cd rapidcheck
+    ```
+
+2. Build RapidCheck (requires CMake to be installed).
+
+    ```shell
+    cmake -DCMAKE_POSITION_INDEPENDENT_CODE:BOOL=true -DRC_ENABLE_BOOST_TEST=ON $(pwd)
+    make && make install
+    ```
+
+3. Configure Bitcoin Core with RapidCheck.
+
+    `cd` to the directory of your local bitcoin repository and run
+    `./configure`. In the output you should see:
+
+    ```shell
+    checking rapidcheck.h usability... yes
+    checking rapidcheck.h presence... yes
+    checking for rapidcheck.h... yes
+    [...]
+    Options used to compile and link:
+    [...]
+      with test     = yes
+        with prop   = yes
+    ```
+
+4. Build Bitcoin Core with RapidCheck.
+
+    Now you can run `make` and should see the property-based tests compiled with
+    the unit tests:
+
+    ```shell
+    Making all in src
+    [...]
+    CXX      test/gen/test_bitcoin-crypto_gen.o
+    CXX      test/test_bitcoin-key_properties.o
+    ```
+
+5. Run the unit tests with `make check`. The property-based tests will be run
+   with the unit tests.
+
+    ```shell
+    Running tests: crypto_tests from test/crypto_tests.cpp
+    [...]
+    Running tests: key_properties from test/key_properties.cpp
+    ```
+
+That's it! You are now running property-based tests in Bitcoin Core.