added zstdgrep documentation

presenting `zstdgrep` limit regarding dictionary compression
with workaround recommended by @tobwen (#1268)

diff --git a/programs/README.md b/programs/README.md
index 2833875e5dce19c70333ade3fceb56d2dd285d66..22a00409c1cf170f477f8d667a3583a36432894c 100644 (file)
--- a/programs/README.md
+++ b/programs/README.md
@@ -185,7 +185,7 @@ version is less than `128 MiB`).
 
 Compression Speed vs Ratio | Decompression Speed
 ---------------------------|---------------------
-![Compression Speed vs Ratio](../doc/images/ldmCspeed.png "Compression Speed vs Ratio") | ![Decompression Speed](../doc/images/ldmDspeed.png "Decompression Speed")
+![Compression Speed vs Ratio](https://raw.githubusercontent.com/facebook/zstd/v1.3.3/doc/images/ldmCspeed.png "Compression Speed vs Ratio") | ![Decompression Speed](https://raw.githubusercontent.com/facebook/zstd/v1.3.3/doc/images/ldmDspeed.png "Decompression Speed")
 
 | Method | Compression ratio | Compression speed | Decompression speed  |
 |:-------|------------------:|-------------------------:|---------------------------:|
@@ -208,10 +208,24 @@ The below table illustrates this on the [Silesia compression corpus].
 [Silesia compression corpus]: http://sun.aei.polsl.pl/~sdeor/index.php?page=silesia
 
 | Method | Compression ratio | Compression speed | Decompression speed  |
-|:-------|------------------:|-------------------------:|---------------------------:|
-| `zstd -1`   | `2.878`   | `231.7 MB/s`  | `594.4 MB/s`  |
-| `zstd -1 --long` | `2.929` | `106.5 MB/s` | `517.9 MB/s` |
-| `zstd -5`  | `3.274`    | `77.1 MB/s`  | `464.2 MB/s`  |
-| `zstd -5 --long` | `3.319` | `51.7 MB/s` | `371.9 MB/s` |
-| `zstd -10` | `3.523`    | `16.4 MB/s`   | `489.2 MB/s`  |
-| `zstd -10 --long`| `3.566` | `16.2 MB/s` | `415.7 MB/s`  |
+|:-------|------------------:|------------------:|---------------------:|
+| `zstd -1`        | `2.878` | `231.7 MB/s`      | `594.4 MB/s`   |
+| `zstd -1 --long` | `2.929` | `106.5 MB/s`      | `517.9 MB/s`   |
+| `zstd -5`        | `3.274` | `77.1 MB/s`       | `464.2 MB/s`   |
+| `zstd -5 --long` | `3.319` | `51.7 MB/s`       | `371.9 MB/s`   |
+| `zstd -10`       | `3.523` | `16.4 MB/s`       | `489.2 MB/s`   |
+| `zstd -10 --long`| `3.566` | `16.2 MB/s`       | `415.7 MB/s`   |
+
+
+#### zstdgrep
+
+`zstdgrep` is a utility which makes it possible to `grep` directly a `.zst` compressed file.
+It's used the same way as normal `grep`, for example :
+`zstdgrep pattern file.zst`
+
+`zstdgrep` is _not_ compatible with dictionary compression.
+
+To search into a file compressed with a dictionary,
+it's necessary to decompress it using `zstd` or `zstdcat`,
+and then pipe the result to `grep`. For example  :
+`zstdcat -D dictionary -qc -- file.zst | grep pattern`