bricks: details version and modifications

* README, spot/bricks/README: Here.

spot/bricks/README

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+Bricks is a collection of useful C++ code, organised in standalone
+header files. By default, the code uses the latest stable C++ standard.
+Bricks is publicly available at:
+       https://github.com/paradise-fi/divine/tree/master/bricks
+
+This directory only contains the necessary subset to have a
+multithreaded hashmap in Spot. The integration with Spot requires
+modifications of the original files:
+    - prefer <spot/misc/clz.hh> to builtin clz
+    - use SPOT_FALLTHROUGH when required
+
+In addition to the previous modifications, and to please both GCC and
+Spot compilation options, the following changes have been made:
+    - add missing override
+    - add missing = default
+    - add missing default values
+
+
+This directory contains the current version of Bricks as of Divine 3.3.2.