Enable miplib datasets which was disabled and also update README (NVIDIA#414)

This dataset was disabled in PR NVIDIA#335 since the website was down, the services have been restored as of now, so enabling this back.

And also adding details to README to benchmark mittleman and miplib.

## Issue
NVIDIA#335

Authors:
  - Ramakrishnap (https://github.com/rgsl888prabhu)

Approvers:
  - Rajesh Gandham (https://github.com/rg20)

URL: NVIDIA#414

Author: rgsl888prabhu

benchmarks/README.md

@@ -1,3 +1,26 @@
 # Benchmarks Scripts
 
-This directory contains the scripts for the benchmarks.
+This directory contains the scripts for running benchmarks. Currently it supports running benchmarks on LP with and MILP.
+
+
+# Linear Programming Benchmarking
+
+
+- Mittelmann LP benchmark
+
+```bash
+benchmarks/linear_programming/utils/benchmark_lp_mittelmann.sh
+```
+
+- MIPLIB Benchmark
+
+```
+mkdir miplib_data
+mkdir miplib_result
+wget https://miplib.zib.de/downloads/benchmark.zip -O miplib_data/benchmark.zip
+unzip miplib_data/benchmark.zip -d miplib_data
+find miplib_data -name "*.gz" -exec gunzip {} \;
+find miplib_data -name "*.gz" -delete
+
+benchmarks/linear_programming/run_mps_files.sh --path miplib_data/ --write-log-file --log-to-console false --output-dir miplib_result --time-limit 600 --presolve t > miplib_result/output.log 2>&1
+```