Update README.md

1_taxonomic_profiling_and_vizualization/2_practical_quality_control/README.md

 # Practical: Quality control and read trimming
 
+Objective of the practical:
+- Familiarise with fastq files, how to look at them and what they represents;
+- Learn how to understand if the reads in your files are of good quality;
+- Learn how to remove and trim low quality reads.
+
 Required tools:
 - [trimmomatic](http://www.usadellab.org/cms/?page=trimmomatic) (install with [conda](https://anaconda.org/bioconda/trimmomatic))
 - [FastQC](https://www.bioinformatics.babraham.ac.uk/projects/fastqc/) (install with [conda](https://anaconda.org/bioconda/fastqc))