add figures to Internals docs

Author: mlin

docs/gri.png

@@ -2,10 +2,14 @@
 
 ## Compression layer
 
+![Compression layer figure](sqlite_zstd_vfs.png)
+
 *To be written; for now see [sqlite_zstd_vfs](https://github.com/mlin/sqlite_zstd_vfs)*
 
 ## Genomic Range Index
 
+![GRI figure](gri.png)
+
 The GenomicSQLite GRI is a conventional multi-column B-tree index, organized so that feature overlap with any length distribution is detectable using a series of SQL "between tuples" queries. 
 
 1. Partition the features into "levels" according to their length. Each level *L* for 0 &le; *L* &lt; 16 consists of all features with 16<sup>L-1</sup> &lt; length &le; 16<sup>L</sup>. (Level 0 has features of length 0 or 1.)