Add guide for running bigdl-example using ipex-llm libs (#28)

* add guide

* update

Author: hzjane

docs/bigdl-llm-upgrade.md

@@ -0,0 +1,33 @@
+# bigdl-llm upgrade guide
+## Upgrade bigdl-llm to ipex-llm
+Need to uninstall bigdl-llm and install ipex-llm first.
+```bash
+pip uninstall -y bigdl-llm
+pip install --pre --upgrade ipex-llm[all] # for cpu
+pip install --pre --upgrade ipex-llm[xpu] -f https://developer.intel.com/ipex-whl-stable-xpu # for xpu
+```
+## Run bigdl-llm example code
+Choose a way to modify the example code to use.
+
+### 1.Run bigdl-llm example code with ipex-llm in compatible mode
+Minimal change: Use `BIGDL_COMPATIBLE_MODE` to run, need to add `import ipex_llm` at the 
+beginning of code with bigdl-llm examples.
+```python
+# need to add before import bigdl.llm
+import ipex_llm
+from bigdl.llm.transformers import AutoModelForCausalLM
+model = AutoModelForCausalLM.from_pretrained(model_path,
+                                             load_in_4bit=True,
+                                             trust_remote_code=True)
+```
+
+### 2.Upgrade bigdl-llm example code to ipex-llm
+Need to replace all `bigdl.llm` with `ipex_llm`.
+```python
+#from bigdl.llm.transformers import AutoModelForCausalLM
+from ipex_llm.transformers import AutoModelForCausalLM
+model = AutoModelForCausalLM.from_pretrained(model_path,
+                                             load_in_4bit=True,
+                                             trust_remote_code=True)
+```
+