Main README: Add "flex" to packages needed for development
Main README: Add "flex" to packages needed for development

opencog/link-grammar,linas/link-grammar,ampli/link-grammar,linas/link-grammar,opencog/link-grammar,ampli/link-grammar,ampli/link-grammar,linas/link-grammar,ampli/link-grammar,opencog/link-grammar,opencog/link-grammar,linas/link-grammar,opencog/link-grammar,linas/link-grammar,opencog/link-grammar,opencog/link-grammar,opencog/link-grammar,ampli/link-grammar,linas/link-grammar,ampli/link-grammar,opencog/link-grammar,linas/link-grammar,linas/link-grammar,ampli/link-grammar,linas/link-grammar,ampli/link-grammar,ampli/link-grammar