Add r2yara generartor section

src/r2yara/rules_generator.md

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+## Rule generator
+
+r2yara allows the creation of YARA rules inside radare2 with the `yrg` group of commands:
+
+```console
+[0x00000000]> yrg?
+Usage: yrg [action] [args..]   load and run yara rules inside r2
+| yrg-          delete last pattern added to the yara rule
+| yrg-*         delete all the patterns in the current rule
+| yrgs ([len])  add string (optionally specify the length)
+| yrgx ([len])  add hexpairs of blocksize (or custom length)
+| yrgf ([len])  add function bytepattern signature
+| yrgz          add all strings referenced from current function
+```
+
+The current state of the YARA rule can be displayed with the `yrg` command:
+
+```console
+[0x00000000]> yrg
+WARN: See 'yrg?' to find out which subcommands use to append patterns to the rule
+rule rulename : test {
+  meta:
+    author = "user"
+    description = "My first yara rule"
+    date = "2024-10-22"
+    version = "0.1"
+}
+```
+
+Let's assumed we have found an interesting string during reversing:
+
+```console
+[0x00132700]> ps
+expand 32-byte k
+```
+
+To add into this pattern to the current YARA rule, the command `yrgs` can be used together with the length of the pattern to add:
+
+```console
+[0x00132700]> yrgs 16
+[0x00132700]> yrg
+rule rulename : test {
+  meta:
+    author = "sylvain"
+    description = "My first yara rule"
+    date = "2024-10-22"
+    version = "0.1"
+  strings:
+    $ = "expand 32-byte k"
+  condition:
+    all of them
+```
+
+The rule parameters can be changed in the configuration. For example to change the name of the rule, use the following command:
+
+```console
+[0x00132700]> e yara.rule = Salsa20
+```
+
+Once the rule is ready, it can be added to other active YARA rules:
+
+```console
+[0x00132700]> yr+
+INFO: Rule successfully added
+[0x00132700]> yrl
+Salsa20
+```