update dependency requisite to solve compatible issues.

1. update README.md file with descriptions regarding model training for non-model organisms.
2. add a constraint of sequence length allowed for lncRNA prediction with secondary structure features because RNAfold can't predict secondary structure for a very long sequence.

1. Update the requirement of scikit-learn and xgboost to the latest version.
2. Update the default model to allow it to work with the latest scikit-learn and xgboost module.

1. update library requirement for scikit-learn
2. add examples for FASTA format

Update instructions for ViennaRNA installation

1. update the version of required side packages.
2. update the default models
3. fix some bugs caused by newer version of side packages.