added README for misc scripts

misc/README.md

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+The [K8 Javascript engine][k8] is needed to run Javascripts in this directory.
+Precompiled k8 binaries for Mac and Linux can be found at the [K8 release
+page][k8].
+
+* [paf2aln.js](paf2aln.js): convert PAF to [MAF][maf] or BLAST-like output for
+  eyeballing. PAF has to be generated with minimap2 option `-S`.
+
+* [mapstat.js](mapstat.js): output basic statistics such as the number of
+  non-redundant mapped bases, number of split and secondary alignments and
+  number of long gaps. This scripts seamlessly works with both SAM and PAF.
+
+* [pbsim2fa.js](pbsim2fa.js): convert reads simulated with [PBSIM][pbsim] to
+  FASTA and encode the true mapping positions to read names.
+
+* [pbsim-eval.js](pbsim-eval.js): evaluate mapping accuracy for FASTA generated
+  with [pbsim2fa.js](pbsim2fa.js). This script only works with PAF. For SAM,
+  please run the following instead:
+  ```sh
+  k8 sam2paf.js -p aln.sam | k8 pbsim-eval.js /dev/stdin
+  ```
+
+* [sam2paf.js](sam2paf.js): convert SAM to PAF.
+
+[k8]: https://github.com/attractivechaos/k8
+[k8bin]: https://github.com/attractivechaos/k8/releases
+[maf]: https://genome.ucsc.edu/FAQ/FAQformat#format5
+[pbsim]: https://github.com/pfaucon/PBSIM-PacBio-Simulator