Add a documentation page for the runtime variables

docs/runtime_variables.md

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+## Runtime variables
+
+OpenBLAS checks the following environment variables on startup:
+
+* **OPENBLAS_NUM_THREADS=**   the number of threads to use (for non OpenMP-builds of OpenBLAS)
+* **OMP_NUM_THREADS=**        the number of threads to use (for OpenMP builds - note that setting this may also affect any other OpenMP code)
+* **OPENBLAS_DEFAULT_NUM_THREADS=**   the number of threads to use, irrespective if OpenBLAS was built for OpenMP or pthreads
+
+* **OPENBLAS_MAIN_FREE=1**"   this can be used to disable automatic assignment of cpu affinity in OpenBLAS builds that have it enabled by default
+* **OPENBLAS_THREAD_TIMEOUT=**  this can be used to define the length of time that idle threads should wait before exiting
+* **OMP_ADAPTIVE=1**      this can be used in OpenMP builds to actually remove any surplus threads when the number of threads is decreased
+* 
+DYNAMIC_ARCH builds also accept the following:
+* **OPENBLAS_VERBOSE=**       set this to "1" to enable a warning when there is no exact match for the detected cpu in the library
+*                             set this to "2" to make OpenBLAS print the name of the cpu target it autodetected
+* **OPENBLAS_CORETYPE=**      set this to one of the supported target names to override autodetection, e.g. OPENBLAS_CORETYPE=HASWELL
+
+
+
+Deprecated variables still recognized for compatibilty:
+* **GOTO_NUM_THREADS=**    equivalent to **OPENBLAS_NUM_THREADS**
+* **GOTOBLAS_MAIN_FREE**   equivalent to **OPENBLAS_MAIN_FREE**
+* **OPENBLAS_BLOCK_FACTOR**   this applies a scale factor to the GEMM "P" parameter of the block matrix code, see file driver/others/parameter.cen