[Doc] Add benchmarking notes

doc/benchmarking.md

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+Benchmarking
+============
+
+Bitcoin Core has an internal benchmarking framework, with benchmarks
+for cryptographic algorithms such as SHA1, SHA256, SHA512 and RIPEMD160. As well as the rolling bloom filter.
+
+After compiling bitcoin-core, the benchmarks can be run with:
+`src/bench/bench_bitcoin`
+
+The output will look similar to:
+```
+#Benchmark,count,min,max,average
+RIPEMD160,448,0.001245033173334,0.002638196945190,0.002461894814457
+RollingBloom-refresh,1,0.000635000000000,0.000635000000000,0.000635000000000
+RollingBloom-refresh,1,0.000108000000000,0.000108000000000,0.000108000000000
+RollingBloom-refresh,1,0.000107000000000,0.000107000000000,0.000107000000000
+RollingBloom-refresh,1,0.000204000000000,0.000204000000000,0.000204000000000
+SHA1,640,0.000909024336207,0.001938136418660,0.001843086257577
+SHA256,256,0.002209486499909,0.008500099182129,0.004300644621253
+SHA512,384,0.001319904176016,0.002813005447388,0.002615700786312
+Sleep100ms,10,0.205592155456543,0.210056066513062,0.104166316986084
+Trig,67108864,0.000000014997003,0.000000015448112,0.000000015188842
+```
+
+More benchmarks are needed for, in no particular order:
+- Script Validation
+- CCoinDBView caching
+- Coins database
+- Memory pool
+- Wallet coin selection