Add some documentation for web browser rewriting

doc/browser-demo.md

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+E9Tool/E9Patch Web Browser Guide
+================================
+
+This is a short demo for instrumenting modern web browsers using
+E9Tool/E9Patch.
+
+---
+## Instrument Firefox
+
+Modern versions of Firefox should be straightforward to instrument.
+Below is a basic example:
+
+0. Install E9Tool/E9Patch:
+
+   * [https://github.com/GJDuck/e9patch/releases](https://github.com/GJDuck/e9patch/releases)
+
+1. Download
+[Firefox](https://ftp.mozilla.org/pub/firefox/releases/108.0.2/linux-x86_64/en-US/firefox-108.0.2.tar.bz2):
+
+            $ wget https://ftp.mozilla.org/pub/firefox/releases/108.0.2/linux-x86_64/en-US/firefox-108.0.2.tar.bz2
+
+   For this demo we use Firefox version (108.0.2).
+   Other versions of Firefox may also work (untested).
+2. Extract Firefox:
+
+            $ tar xvfj firefox-108.0.2.tar.bz2
+            $ cd firefox
+
+3. Compile the instrumentation.
+   For this demo we use `counter.c`:
+
+            $ e9compile /usr/share/e9tool/examples/counter.c
+
+4. Instrument the `libxul.so` binary.
+   Note that `libxul.so` is the main Firefox binary
+   (`firefox-bin` is mostly a wrapper that dynamically loads `libxul`).
+   In this example, we will insert `counter` instrumentation for each jump
+   instruction:
+
+            $ mv libxul.so libxul.orig.so
+            $ e9tool -M jmp -P 'entry()@counter' -c 5 -o libxul.so ./libxul.orig.so 
+
+   Here:
+
+   * "`-M jmp`" matches all jump and conditional jump instructions.
+   * "`-P entry()@counter`" inserts a call to the `entry()` function defined
+      in `counter.c` for each matching instruction.
+   * "`-c 5`" tells E9Patch not to aggressively compress the output binary.
+      Since the input binary is quite large (~145MB), the output binary may use
+      more mappings beyond the system default.
+      This option reduces the number of mappings in exchange for a larger
+      output binary.
+   * "`-o libxul.so`" specifies that the output binary should be called
+      `libxul.so` (replacing the original).
+   * "`libxul.orig.so`" is the input binary (renamed from `libxul.so`).
+
+5. Run Firefox with the instrumented `libxul.so`:
+
+            $ ./firefox
+            count = 1000000
+            count = 2000000
+            count = 3000000
+            count = 4000000
+            ...
+
+Notes:
+
+* The `counter` instrumentation is not thread safe, so the counts will be
+  inaccurate for multi-threaded programs like Firefox.
+  However, this is just an example for demonstration purposes.
+* The instrumented Firefox should be stable and behave the same as the
+  original, except for being slower and printing count information.
+* Other kinds of instrumentation/rewriting is possible.
+  Please see the [E9Tool User
+  Guide](https://github.com/GJDuck/e9patch/blob/master/doc/e9tool-user-guide.md) for more information.
+
+---
+## Instrument Google Chrome?
+
+It is also possible to instrument Google Chrome using E9Tool/E9Patch.
+However, for modern versions of Chrome, this can be troublesome:
+
+1. Chrome frequently uses data-in-code; and
+2. Chrome seems to copy some code to different locations at runtime.
+   This breaks some of the basic assumptions for static binary rewriting.
+
+It is possible to manually exclude affected regions from rewriting (see
+E9Tool's `-E` option).
+However, this process is manual, and depends on the specific Chrome version.
+
+For older versions of Chrome (circa 2020), it is possible to successfully
+rewrite Chrome by excluding all code before the "`ChromeMain`" symbol:
+
+        $ e9tool -E '.text..ChromeMain' ...
+
+Here:
+
+*  "`-E '.text..ChromeMain'`" tells E9Tool to ignore all code in the
+"`.text`" section before `ChromeMain`, which is usually <3% of all code.
+
+This simple trick no longer works for modern versions of Chrome.
+