docs(README): add plugin screenshots

Add the docs/img/ screenshots and embed one in each plugin's subsection:
palette (deft-mx), radial (deft-radial), acme (deft-acme-select), sam
(deft-sam), and plumb (deft-ollie-plumb). The switch plugin has no screenshot
yet.

README.md

@@ -142,6 +142,8 @@ defaults.
 
 ### `[deft:util] palette` — command palette (M-x)
 
+![The Deft command palette open over a Kate window](docs/img/deft-mx.png)
+
 A Sublime/Emacs-style launcher over **every action currently offered by the
 window**, matched with the orderless `FuzzyRanker` (space-separated tokens in
 any order, with typo tolerance) and ranked by *frecency* (frequency + recency of
@@ -187,6 +189,8 @@ discovered from the active document (VCS root, else its directory).
 
 ### `[deft:util] radial` — radial caret menu (mouse)
 
+![The Deft radial caret menu](docs/img/deft-radial.png)
+
 A multi-level pie menu for actions that **complete in place** (cut, copy, paste,
 comment, case changes…), so the mouse stays at the text.
 
@@ -201,6 +205,8 @@ comment, case changes…), so the mouse stays at the text.
 
 ### `[deft:nineify] acme` — acme editing gestures
 
+![Acme-style bracket/quote pair selection in Kate](docs/img/deft-acme-select.png)
+
 Plan 9 *acme* muscle memory inside Kate. Installed as a filter so it never
 clashes with Kate's own bindings.
 
@@ -212,6 +218,8 @@ clashes with Kate's own bindings.
 
 ### `[deft:nineify] sam` — structural regular expressions
 
+![The Sam structural-regexp tool view in Kate](docs/img/deft-sam.png)
+
 Run Plan 9 *sam* structural-regexp programs against your text — the expressive
 `x/re/`, `g/re/`, `s/re/txt/`, `,` dot-addressing model, not line-at-a-time
 find/replace.
@@ -224,6 +232,8 @@ find/replace.
 
 ### `[deft:nineify] plumb` — Plan 9 plumber
 
+![Plumbing a path from Kate to the Plan 9 plumber](docs/img/deft-ollie-plumb.png)
+
 Make Kate a first-class *plumb* client.
 
 - **Plumb under the caret:** `F2`. Sends the file-path token under the cursor